I believe it is actually the fist two in the public domain
and they would be a wonderful start
especially if they made a crossover beetween tarzan and it, since that book isnt in the public domain they owuld have to write an entirely new story :O
and I am all for new stuff
I guess they could call it Lord of the Earth's Core.
Defenders of the Earth.
They have The Phantom and Flash Gordon already - just add Mandrake and presto!