An 11-year-old girl who was "celebrating a friend’s birthday party" has died after going missing at a water park near Windsor.
Police investigators said they were called to Liquid Leisure in Berkshire at 3.55pm on Saturday and, following an extensive search, a girl was located around 5.10pm.
She was rushed to Wexham Park Hospital, but sadly passed away, police said.
One witness, who had been at the park with her children, said the girl was seen going under and "didn't come back up".
"At that point, the lifeguards came running," she told Sky News.
"She was with a group of friends for a girl's birthday," said the mother, who did not want to be named.
"My daughter spoke to the birthday group."
Members of the public who were at the activity park were asked to call her name on the beach, in case she had got out of the water and was back on land.
"But no one found her," the witness said. "The lifeguards were working incredibly hard.”
Another visitor claimed that staff members asked them for goggles so they could look for the girl underwater.
She said: "We were waiting to go on things and lifeguards were running all over the obstacles and looking under them.
Footage from the Datchet park shows emergency services, including fire crews and an air ambulance, at the scene.
Thames Valley Police said officers worked with Royal Berkshire Fire and Rescue Service, Buckinghamshire Fire and Rescue Service, Surrey Fire and Rescue Service and South Central Ambulance Service to respond immediately to the report and an extensive search was commenced, with support from the National Police Air Service.
The girl's death is being treated as unexplained and an investigation has been launched, Thames Valley Police said.
Although formal identification has yet to take place, the girl's next of kin have been informed and they are being offered support by officers.
Local policing area commander for Windsor and Maidenhead, Superintendent Michael Greenwood, said: “My thoughts are with the family and friends of the girl who has died as a result of this tragic incident.
“We are in the early stages of investigating this incident to understand the full circumstances.
“There was a swift response from all emergency services and, following an extensive search of the lake, the girl, who was 11, was located at around 5.10pm and taken to hospital but sadly died."
Superintendent Greenwood said it had "been an extremely traumatic and upsetting incident for all involved".
“I am aware that several members of the public entered the lake shortly after the girl got into difficultly, but were unable to locate her," he said.
“I would like to commend them for their courage and bravery.
“My sincere condolences are with the girl’s family and friends, and I would ask that their privacy is respected at this incredibly traumatic and distressing time for them.”
A spokesperson for Liquid Leisure said: “Our thoughts and prayers are with the family, friends and all those affected by this desperately tragic and upsetting incident.
"As Thames Valley Police are in the early stages of an investigation it would not be right for us to add further comment, but we will continue to fully support and assist them throughout the process.”
The waterpark is around 20 miles from central London and is located just 10 minutes from Heathrow airport.
Liquid Leisure says on its website that visitors can take part in activities such as wakeboarding or attempting to cross floating obstacle courses.