The Elephant Foot. Radioactive material from the Chernobyl meltdown

The elephant’s foot is molten sludge. It’s ‘corium’. As the reactor system breaks down, the rods of nuclear fuel are no longer cooled, and their decay heat is such that they will get really, really, really hot. So hot, that they’ll melt into a lava-like substance and will also melt (and mix with) the various bits of material that forms the lining of the fuel rod when the reaction is running properly.

This lava-like sludge remains naturally hot without any energy input and will just start melting through everything.

That’s what you are looking at now.