"You go first" -Lead spelunker for a troop of Bleardio Beardsman's spelunking relic hunters.
The ravines that scar the surface of the Avantsphere (like Ro'Ro) occasionally crack further with the movement of the tectonics and create new openings and caves that extend down into the sphere. The brave, crazy, and crazy brave occasionally--with varying levels of preparation--attempt to enter the world below and see what lies beneath the Ava.
That mystifying land is known as the Undersphere.
Do I even want to know what's down there?
Operations like the excavation prgoram run by Bleardio Beardsman and privately financed relic hunters said trained spelunkers into the shell of the sphere to dig around its innards and see what there is to find, usually with the goal of finding artifacts left over from the Old World. With so much time passing in the land sealed from the surface, they stand to often find more than dusty relics, but occassionally stumble upon hidden realms living in parallel to their own--as vexign as they can be deadly.