A police spokesperson said: “Around 6.10pm on Sunday (July 18), police received a report of a man missing at Ardingly Reservoir.”
The 49-year-old man had not been seen since going swimming with friends in the afternoon, police said.
They added: “Extensive searches were carried out on Sunday, with the assistance of the National Police Air Service helicopter, and are continuing this morning (Monday, July 19).”
A West Sussex Fire & Rescue Service spokesperson said: “We were called by Sussex Police to support with searching Ardingly Reservoir after a report of a person entering the water.
“Our search was suspended at just before 10pm because of nightfall.
“Crews from Crawley and Turners Hill fire stations were in attendance as well as our Technical Rescue Unit.”
These photos show emergency services at the scene.