I hope you had a wonderful weekend and a pleasant Presidents' Day holiday.

I am pumped to be back here with you to start this week off strong.

We're gonna begin with the latest news out of Ukraine.

This week, President Biden made an unexpected visit to Kyiv.

It was his first visit since Russia launched its full-scale invasion of Ukraine almost a year ago.

Here's part of what the President had to say about this trip.

That dark night one year ago, the world was literally, at the time, bracing for the fall of Kyiv.

One year later, Kyiv stands and Ukraine stands.

Democracy stands.

The Americans stand with you and the world stands with you.

Now, there's a long history of American presidents visiting war zones.

This trip was unprecedented in modern history, however, because the American president traveled to an active war zone without a large US military presence.

President Biden said his trip was to reaffirm the, quote, "unwavering support that the United States holds for Ukraine".

Earlier this month, the United States said Russia has committed crimes against humanity in Ukraine.

That was an escalation of what they'd said previously.

Russia called the accusation "an excuse for Washington to further the conflict".

During his trip, President Biden also announced new military assistance for Ukraine and incoming sanctions against Moscow.

We'll keep you informed on any more updates from the President's trip as well as new information on the ongoing war overseas

And, over the weekend, the secretive nation of North Korea launched a series of missile tests off the coast of the Korean peninsula.

The country says the missiles have enough range to hit the United States, but the US has said this weekend's events, they don't pose any immediate threat to US personnel or territory.

The tests do, however, highlight the destabilizing impact of North Korea's illegal missile program.

North Korean leader Kim, Jong-un's sister warned of more threats to come unless the United States halts military drills with South Korea.

More now from Paula Hancocks, CNN international correspondent based in Seoul, South Korea.

It has been a busy few days on the Korean peninsula.

North Korea, on Saturday, launching a significant missile; on Sunday and Monday, we have seen tit-for-tat retaliation from both sides.

North Korea and also the US and South Korea on the other side.

So, what we saw on Saturday, Pyongyang admitted a day later, was an intercontinental ballistic missile.

Now, these, they claim, can hit mainland United States, and they are the ones that do concern Washington.

We heard from Kim, Yo-jong, Kim, Jong-un's sister, that this actually shows that they have missile re-entry technology.

Now, it's very difficult to know if that is, in fact, the case, but that is what Pyongyang is claiming at this point.

So, what we saw on Sunday then, just one day later, was a concerted show of force by the US and South Korean air force⏤air forces.

They flew sorties over the Korean peninsula, including at least one B-1 bomber, escorted by South Korean fighter jets.

And then, in response to that, we saw this Monday morning from Pyongyang two launches.

Now, they say that it was a super large multiple rocket launcher exercise in retaliation for what they saw the day before, so, this is really a latest cycle of tit-for-tat retaliation.

But certainly, the Saturday launch was the significant one.

But we're not expecting the tensions or this tit-for-tat retaliation to to diminish any time soon because Pyongyang says it is doing so because the US and South Korea are carrying out joint military drills.

And if they continue to do that, then they will continue to respond.

Now, we know that this week at the Pentagon, there will be a nuclear tabletop drill between the US and South Korea,

and then, next month, there will be a larger joint military drill between the two countries.

Up next, we're traveling to Canada in search of zombies, but not the type of zombies you'd typically think of.

Ever heard of zombie fires?

I hadn't, either, until now.

They're essentially fires that were never totally put out at the end of a fire season,

and they burn underground, threatening to come back from beneath the surface at any moment to wreak havoc once more.

We'll hear more now from an international team of researchers who were on the hunt for the next fire that could be lurking beneath the earth's surface.

In the remote boreal forests of Canada's Northwest Territories, and Rebecca Scholten, along with an international team of researchers, is on the hunt for signs of a rare and destructive phenomenon.

So, the first thing you notice when you get to a site that has had a zombie fire is that a lot of these trees have fallen over, and that is because of the underground burning that is happening.

In 2015, this region of boreal forest was the scene of an overwintering fire, also known as a zombie fire.

These are rare, so we don't have any footage of them, but they can look just like these regular forest fires, except they're back from the dead.

So, these are fires that are not extinguished at the end of a fire season, but, instead, they smolder deep into the organic soil layer.

And when the snow comes, the snow kind of protects them from the adverse winter conditions,

and that makes it possible for them to smolder all throughout winter.

When the snow melts and there is dry fuels available again, these fires come up to the surface again and start a new flaming forest fire.

In 2021, Scholten published the first-ever scientific study to detect zombie fires using satellites and reports from local fire managers as part of her Ph.D.

For example, in the summer of 2015, a fire in Southwest Alaska blazed across 26 square miles.

Winter came and snow covered the fire site.

But the following spring, the fire returned along the old burn scar, evidence, Scholten says, of a zombie fire.

By burning through the soil and the roots, Zombie fires can be more damaging to the forest than regular fires, making it harder for them to recover.

But that's not all.

These boreal soils store huge amounts of carbon, so burning them releases it as carbon dioxide into the atmosphere.

Zombie fires are rare, but Scholten found they are increasing due to climate change.

So, I think zombie fires are a very good poster child for what is happening to arctic wildfires in general.

So, we see fire regimes are intensifying, and this has very important impact on the ecosystems that, of course, has an impact on our climate.

And that is why I think everyone in the world should care about these fires.

10-second Trivia.

Which of these words comes from a Latin term that described a wind from the north?

Zephyr, bluster, boreal, or arboreal?

Zephyr was a west wind, but when you're talking about a north wind, you're speaking in boreal terms.

Oh, yeah, it's going all the way.

[This erupting aurora was caught on camera in Alaska.]

Oh, yeah, right above; wow, look at how bright that is.

[Particles from explosions on the sun interact with atmospheric gases, creating the mesmerizing light displays.]

[Vincent Ledvina, a doctoral student in space physics, has been photographic polar lights for years.]

All right, today's story getting a 10 out of 10, a shark attack.

Only this time, it's the shark getting attacked by a dog.

That's right, a big old hammerhead shark.

One boater said it was probably about 12 feet long, just minding its business in the Bahamas until a dog spots the monster cruising beneath the surface, and⏤

Cannonball! The dog jumps into the water and starts chasing the shark away.

The people on the tour boat were screaming in disbelief, in horror.

The dog is the pet of the caretaker who manages the private island.

Pardon my "shark-asm" little doggy, but something tells me you're one "jawsitively" lucky pup!

The boaters were relieved when the dog safely swam back to the shore unharmed.

Fun fact for the day.

Did you know that baby sharks are called "pups"?

Learn something new every day.

Shout-out time now; Hudsonville High School in Hudsonville, Michigan, we see you.

Also, shout-out to Mr. Ben Lewis at Brenham Middle School in Brenham, Texas.

He challenged his students to interview a military veteran, an immigrant, and an American citizen over 60 years old

to learn about different perspectives and to gain better understanding of what it means to be an American.

He turned the interviews into this book; proceeds going to help deserving military veterans.

We salute you, Mr Lewis, and to all of the students who sent me a lovely note with all these signatures, you totally rocked it out.

We'll see you tomorrow, everyone; I'm Coy Wire, and we are CNN10.