A Silurian city was located deep underneath the town of Cwmtaff, Wales. The Silurians who lived there went into hibernation when the Moon came and was captured by the Earth. They were woken in the year 2020 by a human mining operation. Several humans were captured from the surface and brought to the city, including Amy Pond. The Eleventh Doctor and Nasreen Chaudhry discovered the city (TV: The Hungry Earth) and tried to negotiate a settlement with the Silurians.
The city was a vast subterranean world, with cave-like buildings filled with plant life and connected by walkways. The city had a court, a laboratory and an army of warriors in hibernation. A safety measure to prevent invasion involved filling the city with toxic gas and sending the warriors back to suspended animation. This was used to prevent Restac's attempted attack on the humans. The city was returned to dormancy, scheduled to reawaken in 3020. A crack in time and space was found here, containing a piece of the TARDIS inside it. (TV: Cold Blood)