Atlantis

Atlantis was an ancient civilization located on the island of Thera. It had flourished due in part to the Kronos Crystal.

Circa 1500 BC, 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 (DW: The Underwater Menace). The Dæmons later claimed they destroyed Atlantis. (DW: The Dæmons)

In 1970, Professor Zaroff discovered the surviving Atlanteans and promised to raise their settlement from the ocean floor. He died in the attempt thanks to the Doctor's intervention. (DW: The Underwater Menace)

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