In The Underwater Menace (2 complete, 2 reconstructed), the Doctor, Ben, Polly and new boy Jamie discover Atlantis! Apparently, the ancient lost city is still populated, surviving under the water, sealed away from the world above.
While the heroes are offered up as a sacrifice to the Atlantan god, a scientist that the Doctor recognises hatches a plan to raise Atlantis out of the sea. However, he is quite mad and really intends to blow up the whole world or something.
This wasn’t a bad serial, and again allowed the Doctor to dress up in disguise (this is becoming a theme). The city gets flooded and destroyed at the end, so the production was fairly good, with pools of water and stuff. I’d rate this one as passable.