Atlantis

Atlantis was an ancient civilization located on the island of Thera. It had flourished due in part to the Kronos Crystal, which had been given to them by the Dæmons. (PDA: The Quantum Archangel)

Prior to Atlantis' destruction, the civilization had advanced spaceflight capabilities and had reached worlds as far away as Vortis in the Isop Galaxy. (DWA: The Lost Ones)

Circa 1500 BCE, Atlantis was destroyed when the Master released Kronos, despite the Doctor's attempt to intervene. (DW: The Time Monster)

Some Atlanteans survived beneath the ocean, off the Azores. In 1970, Professor Zaroff discovered the surviving Atlanteans and promised to raise their settlement from the ocean floor. This would have destroyed the Earth in the process. He died in the attempt due to the Doctor's intervention. (DW: The Underwater Menace)

Azal later pointed to the destruction of Atlantis as a warning as to what might happen if he considered Earth a failed experiment. (DW: The Dæmons)

Atlantis was an exhibit in the Museum of Things That Don't Exist. (EDA: The Taking of Planet 5)

The Doctor compared Nerva Beacon to Atlantis in the sense that they were both the subject of legend. (DW: The Sontaran Experiment)