The Cave of the Crystals is a massive cave on top of a magma fault, where giant crystals grow and it's so hot and humid inside the cave humans would die after 5-10 minutes exposure. Explorers wear insulated "cold suits" (the orange suits above) but can still stay inside for only about 30-40 minutes at a pop. I saw an initial article about the caves at Good Morning Silicon Valley and chased down some details.
Exploration article from "Angry Planet" (at stormchaser)
National Geographic article on the caves (the source of the above picture) and why the giant crystals formed.
Deadly Crystal Cave video. From National Geographic, it's a bit overdone dramatically (OK, a *lot* overdone). But it still has some interesting information.