The Stagnant Pond was a pub in London. It was managed by Belle Epoch during the Great Fire of London, after which "something happened" and it was left wandering through time. When it eventually returned to London, Belle and the patrons found that they had to return to the pub regularly or time would catch up with them.
In the 1950s, Norton Folgate took Andy Davidson and Gideon Lyme to the Stagnant Pond, ostensibly to keep them safe whilst he dealt with the mushroom creatures. In fact, he had sent them there so that they would get access to the Parnaeum which anchored the pub and use it to charge up Andy's vortex manipulator. As Lyme was supposed to have died, he was able to get through the paradox shield and take the Parnaeum, destroying the pub and killing everyone inside. (AUDIO: Parasite)