The owners of a sand quarry in Bedfordshire say they've had to step up security because people are risking their lives by treating it like a beach.
Groups of up to 60 people have been spotted at Grovebury Quarry in Leighton Buzzard, which is still in operation.
It's thought social media is responsible for spreading the word among local teenagers.
The owners say the problem of trespassers has become so bad they've had to employ extra security, at a cost of thousands of pounds.
Tom Wise, Operations Manager at Garside Sands says "It's become known as "The Beach" on social networking sites. Messages have been encouraging large groups to congregate here and on at least one occasion, there's been evidence that groups of up to fifty or sixty people have been here".
Mr Wise says such behaviour could result in a serious accident or death.
– Tom Wise - Garside Sands
"It is totally irresponsible. They could be buried alive or drown. The sand banks are likely to be unstable and could collapse at any time. Swimmers have no knowledge of how deep the water is or what lies beneath the surface".
Round the clock security patrols are now in place to keep the trespassers out.
Garside Sands says those caught trespassing on the working quarry could face prosecution.