Aridius was a planet originally covered by oceans. Due to a stellar incident, its orbit around its suns turned the planet into a desert over a thousand years. The only remaining species were the amphibious Aridians and the octopus-like Mire Beasts. To control the Mire Beasts the Aridians blew up sections of the city that had been overrun.
The First Doctor visited Aridius, but was tracked by the Daleks, who pursued him in a time machine. (TV: The Chase) The Daleks tried to force the Aridians to hand the Doctor and his companions over, but they escaped in time from the Aridians. One of the Daleks was guarding the TARDIS, but was tricked into falling into the tunnels, enabling the TARDIS crew to escape.
Behind the scenes
According to non-narrative information in DWBIT 41, the Daleks later returned to Aridius, still remembering their encounter with the Doctor there. They wanted revenge on the Aridians and wiped them all out along with the Mire Beasts.