If terraria had realistic caves, it would be much more beautiful, and much more boring. As it stands, terraria's caves were most likely formed by magma, as they do have lava pools and go down to the underworld, which is full of it
Judging by how terraria is on an island, ill say the entire island is a long since inactive volcano, which has been eroded over millions of years
@DoctorMcDerp A cave in Southern Indiana. I don't know the name. But I can now define chert and shale lol
I think this one was primarily limestone. There were some super cool formations and ice age fossils. Was really cool!