It's often geographically divided between Maritime Southeast Asia and Mainland Southeast Asia.

And something weird is happening in Mainland Southeast Asia when you pay closer attention to it on a population density map.

There's a sort of population void that's going on in the center of Southeast Asia that roughly lines up with the borders of Laos and Cambodia.

Indeed, both of these countries have tiny populations in comparison to their immediate similarly sized neighbors.

While Laos only has a small population of 7.9 million people and Cambodia about 17.6 million people,

Thailand to their west has a much larger population of 71.6 million people, while Vietnam to their east has an even bigger population than that at 101.6 million people, making it among the most heavily populated countries in the world.

And then the differences between them in terms of population density are even more stark.

Vietnam has an average population density of about 322 people per square kilometer, making it the population anchor of Mainland Southeast Asia and making it about as densely populated as other crowded countries like Japan and Pakistan.

Thailand's average density is much lower than Vietnam's at about 140 people per square kilometer, which is about the average of the Southeast Asia region as a whole, while Cambodia's density is only about 100 people per square kilometer, and Laos's is way lower at only about 34 people per square kilometer, only about one-tenth the average population density that's seen right next door in Vietnam, making them among the largest differences in population density seen between two bordering countries anywhere in the world.

And Laos's super low population density also keeps getting stranger the more you look at it at first.

Laos has by far the lowest population density seen anywhere in Southeast Asia, and it's the 8th least dense country located on the entire Asian continent.

That's more similar to countries in the deserts and the Himalayas than it is to its own immediate neighbors.

So why does this situation exist?

Why are Laos and Cambodia so much more empty than their immediate neighbors, Vietnam and Thailand?

For one thing, there are several notable geographic differences between these countries, especially in the case of Laos.

With an average nationwide elevation of 710 meters above sea level,

Laos is way, way more mountainous than any other country in Southeast Asia is.

And it's just as covered by dense jungles as well, meaning that the extensive mountain and jungle cover all across Laos makes developing its land more difficult and expensive than elsewhere in the region.

Largely because of all these mountains and jungles,

Laos also has by far the lowest amount of arable land in mainland Southeast Asia as well.

Only 6.2% of Laos's territory is considered to be arable and suitable for agriculture, compared to more than 20% of Vietnam's land, 22% of Cambodia's, and more than 30% of Thailand's land.

And because Laos is also geographically smaller than these other countries are, that means that Laos has only a tiny fraction of the amount of available land for agriculture as these other countries do.

Laos has only about one-third of Cambodia's arable land, one-fifth of Vietnam's arable land, and only about one-tenth of Thailand's massive amount of arable land.

But the availability of arable land in the cover of mountains and jungles does not explain the full picture.

Laos has about as much arable land as Slovakia does, and their populations are fairly comparable.

But Laos also has nearly as much arable land as South Korea does too.

And South Korea still has orders of magnitude more people.

And while Laos is largely covered by mountains, the Chinese province of Yunnan just to the north of Laos is even more mountainous with more than twice the average elevation.

And yet it also has nearly four times the average population density of Laos too.

Then Cambodia is by far the least mountainous, most flat country in mainland Southeast Asia, with an amount of arable land that's not too dissimilar from Vietnam.

And yet the overall density of people in Cambodia is still less than a third of what it is next door in Vietnam anyway.

So there's more going on here.

Part of the answer to the puzzle is that the quality of the arable land that's present in Vietnam is simply much higher than it is in just about anywhere else on the planet.

Out of the top five most agriculturally productive river deltas in the world, Vietnam controls two of them.

The Red River Delta in the north that's home to Hanoi, and the Mekong River Delta in the south that's home to Ho Chi Minh City, which is the second most agriculturally productive river delta in the world behind only the Ganges-Brahmaputra Delta that spans across Bangladesh and India.

The Mekong Delta itself is massive at almost the size of the entire Netherlands, and it's almost as densely populated.

The region is covered in a maze of rivers and canals that bring enormous volumes of nutrient-rich sediment to the area downstream from the Mekong River source high up in the mountains of Tibet.

The Vietnamese government itself estimates that roughly 1 billion cubic meters worth of sediment is deposited by the Mekong River to the delta region every single year, which advances the region by between 60 to 80 meters into the sea every year as well.

All of this constant depositing of nutrient-rich sediment to the Mekong Delta constantly fertilizes the region's fields for agriculture, while the year-round humid and warm temperatures with massive rainfall levels enables farmers to carry out multiple crop cycles a year.

As a result, the Mekong Delta is one of the most productive rice harvesting regions in the world, where farmers can often produce three rice crops annually.

The region produces 60% of Vietnam's total rice harvest, which is why it's also known as Vietnam's rice bowl.

And then on top of that,

Vietnam also controls the fifth most agriculturally productive river delta in the world too up in the north around the Red River Delta.

The historical cradle of the Vietnamese civilization, the Red River Delta has similar advantages to the Mekong River Delta and produces another 20% of Vietnam's total annual rice harvest.

Even though it only makes up 5% of Vietnam's land, 30% of Vietnam's population lives here, and it's home to nearly as many people as in all of Cambodia and Laos combined.

Naturally, nearly 60% of Vietnam's huge population all just live in the flat and agriculturally productive regions around the Mekong and Red River Deltas, while Vietnam also just has a much easier ability to trade with its neighbors as well.

Vietnam has a really long coastline and it's also much closer to China's major population centers than the other Southeast Asian nations are, enabling them greater trade opportunities and better economic potential.

Cambodia has a coastline too, but it's much smaller than Vietnam's is, while Laos is completely landlocked, the only landlocked country in Southeast Asia.

Laos especially is a pretty nerfed country when it comes to its ability to conduct international trade.

Not only is the country landlocked and has no direct access to the sea, but its borders along China, Myanmar, Vietnam, and much of Thailand and Cambodia are all almost totally covered by mountains, further boxing the country in in its ability to conduct trade.

To put the scale of Laos' isolation from its neighbors into perspective for you, its mountainous border with Vietnam is so remote and difficult to access that there is an entire species of megafauna that lives there that went completely undiscovered by mainstream science until the 1990s.

The saola is a large, bovine creature that often weighs around 100 kilograms, and their range is believed to extend across the remote border region between Laos and Vietnam.

Despite this, and despite tens of millions of people living in Vietnam nearby, the saola to date is the most recent living megafauna to ever be discovered by mainstream science, being only first reported on in 1992, and only first photographed and categorized in 1993, with very, very few photographs of it being taken ever since.

Moreover, even though the Mekong River runs for about 400 kilometers through Laos and forms most of its western borders,

Laos cannot use the river for navigation out to the sea like both Vietnam and Cambodia can do because of the existence of the Cone Falls here, basically directly on Laos' far southern border with Cambodia.

The Cone Falls are a massive obstacle to navigation along the Mekong, since it's actually the widest waterfall in the world at nearly 11 kilometers in width from one edge of its multiple channels to the other.

It is by far the largest waterfall in Southeast Asia, and it's the principal reason why the Mekong River has never been fully navigable from the sea all the way into China, and by extension, why Laos cannot use the Mekong River to conduct trade with the outside world through the sea either, while Cambodia and Vietnam further downstream of the falls can, further contributing to Laos' isolation, remoteness, and lack of economic opportunity that attracts people.

This is all why, in terms of GDP per capita,

Laos ranks very low and is among the poorest countries in the world today, with multiple sub-Saharan African countries having higher GDP per capita today.

After civil war-torn Myanmar and tiny Timor-Leste,

Laos has the next lowest GDP per capita in Southeast Asia, which further contributes to Laos' lower population density compared to all of its wealthier neighbors.

To an extent, these geographic differences explain part of the relative emptiness of Laos in particular, but they don't fully explain the relative emptiness of Cambodia, which basically doesn't have any of the geographic handicaps that Laos has.

Cambodia isn't landlocked, it has a major navigable river from the interior to the ocean, it's not covered by mountains that inhibit development, and it has tons of fertile arable land, and yet it still has only about one-third of the population density of Vietnam.

In order to fully explain Cambodia's present demographic situation, we have to dive back into Cambodia's deeply troubled history.

A very long time ago, the primary ethnic group of modern Cambodia, the Khmer, geopolitically dominated Southeast Asia.

The Khmer Empire at its height in the late 13th century controlled most of modern-day Thailand and Laos and southern Vietnam, including the whole of the Mekong Delta.

But shortly afterwards, the Khmer Empire entered into a period of steep decline and collapse during the 15th century around 600 years ago and entered into a sort of dark age.

The Vietnamese then began expanding and colonizing south during this period of Khmer weakness, while the flat ground of Cambodia itself became a frequent battleground between the Vietnamese and the Thai on either side of them, which kept Cambodia's population lower than it otherwise would have been under more stable conditions.

At the turn of the century in 1900, after both Cambodia and Vietnam had been taken over by the French,

Cambodia and Vietnam's populations were even more lopsided than they are now, with colonial Cambodia having only about one-thirteenth of the population of colonial Vietnam.

But of course, both Cambodia and Laos have experienced much more recent catastrophes that have negatively affected their populations and demographics into the present too.

Because the 1960s, and especially the 1970s, are probably the worst decades that either Cambodia or Laos have ever experienced in their entire civilizational histories.

Most of us in the Western world are pretty familiar with America's involvement in Vietnam during the Vietnam War, but a lot fewer of us are familiar with America's simultaneous involvement in Laos and Cambodia during that war that caused enormous destruction as well.

After the French colonial project in Southeast Asia was defeated and collapsed in 1954,

Cambodia and Laos emerged as independent kingdoms, while Vietnam was partitioned in half between a Soviet and Chinese-backed communist state in the north and a US-backed republic in the south.

North Vietnam then quickly began organizing, directing, and arming a communist insurgency in the south that was known as the Viet Cong.

And in order to supply and reinforce them around the heavily militarized north-south border,

North Vietnam began supporting another communist insurgency in neighboring Laos called the Path of Laos.

And then they invaded Laos in 1958 to establish what became known as the Ho Chi Minh Trail, a complex network of roads and trails that they built through the sparsely inhabited mountains and jungles of southeastern Laos.

The existence of the Ho Chi Minh Trail and its use by North Vietnam to supply and reinforce the Viet Cong proved to be a major challenge to the US support of South Vietnam, especially because in 1962, the US signed an international agreement where they promised that they would respect Laos' neutrality and never invade the country.

So, rather than directly invading Laos with ground troops to try and disrupt the Ho Chi Minh Trail that way, the US and the CIA decided to initiate a covert and massive aerial bombing campaign of the country instead that began in 1964.

For the next nine years until the US finally withdrew their troops from South Vietnam, the US Air Force carried out the largest and most overwhelming bombing campaign in human history across Vietnam, Laos, and Cambodia all alike.

In Laos, the US Air Force targeted the Ho Chi Minh Trail and Communist Pha Thanh Lau positions with relentless and indiscriminate airstrikes.

More than 580,000 individual American bombing sorties were carried out across Laos over those nine years, which translated to one American bombing strike taking place in Laos roughly every 8 minutes, 24 hours a day for nine years.

More than 2 million tons of American bombs were dropped on Laos despite its neutrality status.

More bombs than the US dropped throughout the entire Second World War on both Germany and Japan combined, which also makes Laos the most heavily bombed country on a per capita basis in human history.

Virtually all of the bombs that the US dropped on Laos during this time period were also cluster munitions, which detonated in the air shortly before impact and sprayed dozens of miniaturized bomblets across the ground below.

The US used these cluster munitions to try and destroy as much of the Ho Chi Minh Trail's usability to the North Vietnamese forces as possible.

But as a result, the US also dropped an estimated 270 million of those little miniaturized bomblets across Laos' territory, and it's believed that roughly 30% of them failed to detonate upon impact, which means that about 80 million of those small American bomblets have remained hidden and active across Laos to this very day, leaving behind roughly 30% of Laos' territory that is still believed to be infected by massive volumes of unexploded ordnance.

Because it was run covertly by the CIA in order to prevent the American public from ever learning about it, the US government only formally acknowledged its secret bombing campaign in Laos in 1971, seven years after it first began, and they didn't just target militant Vietcong routes either.

They blew up Laotian villages and agricultural land that resulted in massive casualties, while there were numerous reports of American bomber pilots who failed to find suitable targets in Vietnam and then choosing to drop their payloads randomly on Laos instead of returning back to their base with unused bombs.

By the end of the campaign in 1973, it's believed that America's bombing campaign on Laos had killed roughly 200,000 people there, which was an astonishing 10% of Laos' entire population at the time.

Another 400,000 more Laotians were wounded by the bombing campaign too, meaning that nearly one-third of Laos' entire population were either killed or wounded by American bombing, while another 750,000 other Laotians were forced to flee and become refugees by it, all of which has left a significant dent in Laos' demographics that have lasted ever since.

Large amounts of Laos' preciously limited agricultural land still remains contaminated by unexploded bombs that are left over from this conflict, which continues to further restrict Laos' ability to grow food and develop, while more than 20,000 Laotians have also been killed by the unexploded American bombs since the war ended more than half a century ago now.

With around 80 million live cluster munitions still believed to litter the country, experts believe it could still take more than 100 years from now in order to fully clean out the country's infected areas.

Of course, Vietnam itself was also heavily targeted by the U.S. bombing campaign during the war too, and while numerically speaking more Vietnamese were killed during the conflict than Laotians,

Vietnam also suffered a lower percentage loss of its overall demographics than Laos did.

Between 1964 and 1975, Uppsala University in Sweden, which maintains the Armed Conflict Database, estimates that a total of 1,458,059 conflict-related deaths took place across North and South Vietnam among the Vietnamese population, which, if accurate, implies that roughly 4.3% of Vietnam's combined 1964 population would perish during the course of the heavy U.S. involvement in the war, compared to around 10% of Laos' much smaller population.

And then there's the utterly horrific case of Cambodia, which probably suffered the worst decade of its entire civilizational history during the 1970s, in which roughly one-third of the country's entire population would die in one of the most rapid and catastrophic civilizational collapses that has ever been recorded in history, which is a major part of why Cambodia remains more relatively empty than Thailand or Vietnam to this day.

By 1969, at the height of the Vietnam War, the North Vietnamese were using Cambodian territory to supply and support the Viet Cong in the South along multiple fronts.

The Ho Chi Minh Trail that ran through southeastern Laos also extended across eastern Cambodia into South Vietnam, while another lesser-known but even more extensively used North Vietnamese supply route also ran from Cambodia's ports eastward into South Vietnam that became known as the Sihanouk Trail, named after Cambodia's then-king who maintained Cambodia's neutrality, but who also generally tolerated the presence of the communist Vietnamese and their supply routes going through his territory too.

A small homegrown Cambodian communist insurgency existed at this point too, known as the Khmer Rouge, that was steadily growing in the northeast and northwest corners of the country.