It was not a full-fledged TARDIS, but rather a "TARDIS-like" vehicle that was used by the Doctor and his companions to travel around the underwater city of Atlantis.
The TARDIS was actually a time machine, not a car, and was only ever used by the Doctor and his companions to travel through time and space.
It's important to remember that the Doctor's mode of transport has changed throughout the show's long history, and while it is often a car, it is never truly a "car" in the traditional sense.