Phew, is this hot or what.

You shouldn't logically be able to get inside here, the heat is way too high but Solstice has got to the top of the volcano and is using her Bankai to allow people passage into the volcano.

After a small conversation and a request you can leap into the volcano and start exploring the insides, which hold the secret to shutting down the volcano.

Jumping in without protection would be, well, suicide. It seems it's strangely cooler at certain parts and much, much hotter at others.