Carpanthan

The Carpanthans were an advanced amphibious race who lived in the depths of Earth's oceans. In the 1970s, UNIT tested a new depth charge at sea, unintentionally destroying the Carpanthans' city. In retaliation, they wrecked several ships in the same area. The Third Doctor and Liz Shaw investigated, taking a diving bell to the ocean floor, where they discovered the Carpanthans using the sunken ships to form their new city. The Carpanthans captured them and placed them on trial for the destruction of their city by the depth charge, found them guilty and sentenced them to death. However, the Doctor convinced them that he would never reveal their existence to humanity, and they released them both. (TVC: The Fishmen of Carpantha)