A helicopter has been called in to help search a lake for a swimmer who was reported missing on Wednesday night.
Emergency services were alerted at about 1900 BST that the man had disappeared in Arlesey Lake near Hitchin, Bedfordshire.
A helicopter with a thermal imaging camera has joined police, fire and ambulance crews at the lake, known as the Blue Lagoon.
There were unconfirmed reports the man was missing after a boating accident.
Four years ago three children drowned at the lake when a car driven by an unqualified driver tipped over a cliff into the water.
Courtney Barker, 23, was convicted in 2002 of the manslaughter of Imran Khan, aged six, Aaron Khan, five, and Sharla Blair, three.
The tragedy sparked a big campaign for tighter safety precautions but safety barriers and warning signs round the lake have been repeatedly vandalised.
Two years ago police and people living nearby began started Lagoon Watch to try and stop the damage.
Since the hot weather at the weekend three people have drowned in the area - one man in a lake in Stevenage, another in a lake in Milton Keynes and a third in an outdoor pool in Hitchin.