Subtitles section Play video Print subtitles Narrator: From bonnethead sharks to big old Komodo dragons, more than 11 million fluid specimens live in the basement of the Field Museum in Chicago. Josh: There are 883 frogs in here. Narrator: But why hold on to them all? And why keep them wet? Think of it like a library. Stored this way, wet specimens keep their shape and, in some cases, even DNA -- basically, the closest researchers can get to keeping a live zoo in their labs. Each jar is a book researchers can crack open and learn from, sometimes discovering brand-new species right here on the shelves. But you can't just drop a Komodo dragon straight into a tank of alcohol. The Field Museum has to acquire and painstakingly prepare them so they can be preserved for centuries to come. The Field Museum acquires its specimens in two ways: either through donations, or sometimes researchers go out in the field to strategically euthanize specimens like this common water snake. If you manage to grab one, they do not hesitate to bite. Narrator: Sara's research relies on new and old specimens to see how changes in habitat have affected the species over time. And the first thing she does with a new one is grab its DNA. This is a fairly new step in the process, since DNA wasn't really used the way it is now until the '90s. It's not impossible, but it is much easier to take tissue samples from fresh animals than it is to take them from preserved animals and get really good results. Narrator: She uses scissors and forceps to collect the DNA sample, first sanitizing them by burning away any random DNA so the results aren't mixed. Sara: It does get so hot, though, that then I have to dip it into some ethanol so that it doesn't, essentially, sear the animal as I work on it. Narrator: Sara takes the sample from the inside of the snake, so she doesn't mess up what it looks like on the outside. Sara: The way I'm cutting is so that if somebody comes along, they're still going to be able to count these scales. Narrator: Plus, that's right around where the liver is located. Sara: And there's the liver! Right there. Narrator: It's Sara's favorite tissue to collect for DNA extraction because it dissolves easily. Sara: It turns everything pink. It says "SR 1291." I double-check my tag is "SR 1291." Narrator: Then the DNA goes into these massive liquid-nitrogen freezers with thousands of other DNA samples. Now she's ready for formalin, the liquid that preserves the tissue and keeps a specimen frozen in time. It's sort of like embalming it, just like a person at a funeral home. Narrator: Sara has to keep in mind what info she needs now and what researchers might need in the future, like the sex of the snake. Sara: It's always good to see if, maybe if it's a male snake, if you can find the hemipenes and pop them out. Narrator: Sara will pose the snake so you can see the visible penises from outside the jar without even opening it. And this pose is how it will stay for the rest of its afterlife. The coil doesn't just look snaky ... Sara: There is some art to it. And you can stack lots and lots of them on top of each other in a jar. One, two, three, four. I've got five snakes here, and there's another, probably, six in this jar. Narrator: Last step in this part of the process is tucking it in under a formalin-soaked paper towel. It keeps the snake saturated without having to fill the tub. Sara: Good night, snaky. Narrator: Over a few days, the formalin will set into the tissue, leaving the snake fixed in place. Almost like you're holding a rubber snake. Narrator: Larger animals might need more than a few injections, like this catfish Caleb is working on. Calculating the amount of formalin needed is mostly based on experience and feel. Too little, and your specimen will start to decay and get floppy. Too much, and your specimen will bloat and become disfigured. Caleb: You don't want to make the belly do this because you've pumped it with so much formalin. Narrator: Once Caleb is confident his catfish is sufficiently full, he'll move it into a tank of even more formalin to soak for about a week. Caleb: We're going to add a bit of cheesecloth just to make sure that no parts of it are sitting outside of the formalin. Narrator: After the formalin, the team switches over to alcohol baths for long-term preservation, like with this Komodo dragon. The alcohol is less toxic than formalin, so it's safer for researchers in the long run, and the specimen doesn't change much while it sits in its final resting tank, just the color of the liquid. Josh: Especially large specimens, they'll release a lot of debris and fatty oils that were stored in their body, and it'll leech out into the ethanol, and that causes a lot of the discoloration. It's still doing its job and keeping the animal preserved. Narrator: Most specimens in the wet collection are kept looking as lifelike as possible. But others ... Caleb: We can clear away all of the tissue, stain the bones and stain the cartilage, and we can end up with just a skeleton that we can put under a microscope. Narrator: Extra-small fish have extra-small bones that are difficult to keep track of. So instead of isolating the skeleton, this method keeps it contained but visible inside the body. First step is dyeing the specimen blue. This specific blue dye is attracted to cartilage, and the red dye clings to calcium. A few days for each is typically enough to lock in the dye. The next step is to clear the fish. We use an enzyme called trypsin that digests proteins and break them down, but it leaves the collagen that holds everything together. Narrator: Making the fish completely see-through. Finally, he dyes the bones red. Caleb: One of the advantages of clearing the fish and then putting it into the red dye is you can keep an eye on it to see how dark it's getting. Narrator: The whole process can run a few days to around a month. Done right, and your final product are these almost alien-looking specimens. Caleb: It's kind of like Jell-O, and you store it in glycerin in the end, because glycerin and collagen have the same refractive index, or the way that light passes through. Narrator: These specimens go right into collections alongside all the opaque ones, so researchers can access them when all they want to see is bones and cartilage. Caleb: You can put this under a microscope. You can move bones around and see how one bone moving