Long ago, a village lived in peace. Then the flood came. The inhabitants of the village were fine, but they did not want to risk another flood taking out their village. They moved underground, underneath the risen ocean, and lived somewhat peacefully as miners and fishers.
The villagers are now all gone, and now the city is explored by evil monsters and scientists.

P.S. "Terrenus" is Latin for Underground.

Used Voxelsniper to hollow out the cave.