He said it was an "open wound on the surface of the Earth", where the oceanic crust, usually 6-7km thick (3.7-4.3 miles), was simply not there. "Usually the plates are pulled apart and to fill the gap the mantle underneath has to rise up. As it comes up it starts to melt. That forms the magma," he said. "That's the normal process. Here it has gone awry for some reason.
"“Through the fathomless deeps of space swims the star turtle Great A’Tuin, bearing on it’s back the four giant elephants who carry on their shoulders the mass of the Discworld. A tiny sun and moon spin around, on a complicated orbit to induce seasons, so this is probably the only place in the Multiverse where it is sometimes necessary for an elephant to move a leg to allow the sun to go past.
Exactly why it is this way, may never be known. Possibly the Creator of the universe got bored with all the usual business of axial inclination, albedos and rational velocities, and decided to have bit of fun.”