 Good morning, John. In the year 2000, Juan and Pedro Sanchez, two brothers in Mexico, were drilling down in a mine that, like, a lead and silver mine. It was hot down there, like, 130 degrees Fahrenheit. It was, I imagine, unpleasant and dangerous work. And mines are always contending with a lot of different hazards. One of them, though, is water, because mines are underground, and water goes down. People and equipment both work best when they are not submerged in 130-degree water. So mines almost always have huge pumps that remove a tremendous amount of water from the ground, so that people and machines can work down there. Well, in this mine in Mexico, something magnificent had been uncovered by those pumps. With that water taken out, those brothers drilled down into a giant cave, full of some of the largest crystals ever discovered on Earth. Now, an important thing to know, you cannot go visit these crystals. One, because the mine has since stopped pumping the water out, and so that cavern is now full of water again. But maybe, more importantly, the scientists who did get a chance to go down into this cave, they had to wear some very special suits. And those suits were designed to do two things. One, keep them cool, because not only is it 130 degrees Fahrenheit, that's 55 Celsius, it was also extremely humid, because, like, there's a lot of water down there, and all that water was evaporating into that very hot air. And so it was like 90 to 99% humidity in there, which meant that the water that you sweat can't evaporate from your skin into the air. There's no room left in the air for more water, so you have no system for cooling yourself off, and you would overheat and die. And more terrifyingly, they also had to wear basically scuba equipment. They had to breathe air that they carried around with them. But why? Because the air down there isn't poisonous, there's not like some weird alien spores that are gonna infect their lungs. It turns out that 130-degree air can hold a lot of water, right? So 100% humidity with 130-degree air, that's a lot of water in the air. And when you bring the temperature of air down, it can hold less water, so that water condenses out. That's why lots of water vapor going up into the atmosphere condenses into clouds, when it reaches higher, colder areas of the atmosphere. Yes, that's where clouds come from. Also why a glass of cold iced tea has water conducting on the side of it, because the air hits the side of that glass, cools down, and the water falls out. But when it's 130 degrees and 100% humidity outside, you know what's cooler than the air? The inside of your lungs. So if you didn't have a tank of air on your back, every breath you would take in this cave would condense a little bit more water in your lungs, a little more, a little more, until you drown. Now this did not happen to anyone, they knew that it was a thing, and so they were able to plan around it, but still, that's why this is a really, really, really cool planet. But in a sense, like, we kind of never should have seen that cave, and now that the mine is no longer pumping the water out, we can't see it anymore, it's just down there. And we had this very narrow slice of, like, five years during which we could examine and study these giant crystals. We only got to see those crystals because of a truly bizarre series of coincidences. This reminds me of how in Rome, they've spent 40 years trying to build a single subway line, but every time they dig, they find some other, like, truly irreplaceable, remarkable, archaeological site. There's just so much that remains hidden from us. I told Catherine about these crystal caves, and she, like, exclaimed on our walk home, The Earth is an alien planet! Which is just such a good line, because of course, like, there's nothing less alien than the planet that we evolved to exist on, that we as a species have barely ever left, that sustains us every day. But also, it is, right? Even after all of this inquiry, it remains full of the unknown, and it's so inhospitable to us in everywhere, except the very narrow slice, just at the surface. And yet, as we continue to prove over and over again, these unknowns are knowable. And I know that this doesn't make any, like, philosophical sense, but making those unknowns known, whether they are about avocados and megafauna or giant crystal caves, it just seems like a very good use of time. John, I'll see you on Tuesday.