Circle Sea

The Circle Sea was a large circular sea on Avalon. 500 miles in diameter, the sea had been formed by a meteorite impact. The town of Fluxmouth in Elbyon was located on the northwest coast of the sea. The sea was connected to Fluxford via the River Flux. In 2991, the First Doctor and Ian Chesterton sailed aboard the Merrow across the Circle Sea to the centrally-located Shadow Isles, where Merlin's Helm was located. It contained Porpoises. (PROSE: The Sorcerer's Apprentice)

Behind the scenes
The Circle Sea is also the name of a sea in the fantasy  series by Terry Pratchett.