A good idea... There are no oceans left from global warming- they've all dried into seas of loose sand. These are two possible options based off that idea, if Bite Me likes it.
1) It could just be a sea of sand that hardens on the bottom,
or...
2) Beneath the sea of sand there is water. Deep below, there are great, dark, cold, subterranean caves that hold massive freshwater bodies, warmed on their floors by hotsprings. They hold tons of strange and alien life. This is where the Atlantians reside.
Either way, they would have large metal, closed tight like a submarine, worm-like machines with drills on the noses and wheels with treads that grip to anything, that they can use to travel through both water, rock, and sand.
...And maybe some funny little gnome-like creatures that live deep underground, too...?
What do you think, Bite Me?