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 her 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. The attack was aided by the fact that Bella had infected the spa's systems with the Hopper virus in order to get back at Kane who was her long-absent mother. Though the Doctor managed to restore the systems and repel the attack, due to Bella's subsequent sabotage, the Dregs invaded the spa again which was destroyed by one of Bella's bombs. Nevi and Sylas managed to restore the spa's teleporter, allowing them and Team TARDIS to escape while Kane and Bella remained behind to hold off the Dregs. (TV: Orphan 55)