In one possible timeline where Earth was eventually designated Orphan 55, Tranquility Spa was a leisure holiday hotel located on the orphan planet. Led by Kane, the leisure centre was built near the ruins of Novosibirsk, in former Siberia, and protected from the outside world by an ionic membrane.
When the Thirteenth Doctor and friends visited, the spa came under attack by Dregs. A hole in the ionic membrane revealed to all that Tranquility was in fact a "fakation", surrounded by wasteland, but with picturesque scenery playing on screens at the outer borders. (TV: Orphan 55)