One man has died and four people have been injured after getting "the bends" in Leicestershire.
A RAF helicopter was called to rescue the injured divers
Police said they were called to the incident at the National Diving Centre at Stoney Cove by the ambulance service on Sunday morning.
A centre spokeswoman said four divers suffered decompression sickness as they helped an injured man, who died later in hospital, to the surface.
Their injuries are not thought to be life-threatening.
They have been taken to a hospital in Merseyside for treatment in a recompression chamber.
Police said the five people involved were two instructors, two divers and one rescue diver from a Derbyshire dive school.
Margaret Baldwin, operations manager for the centre, said the qualified divers were being supervised by a private diving school and were taking part in a deep-diving training programme.
She added: "The Stoney Cove rescue team responded instantly and almost immediately other divers in the group brought the distressed diver to the surface.
"The four divers in the group ascended faster than normal and as a result have been taken to a recompression chamber for treatment."
Police said an RAF helicopter was scrambled to rescue the injured divers.
A spokesman for Leicestershire Police said they were not treating the incident as suspicious and will be preparing a file for the coroner.
The centre near Hinckley boasts 13 acres of sheltered water with depths of up to 36 metres and is used by about 60,000 divers each year.