A woman who plunged 60ft over the top of a cliff survived the fall - and was able to call 999 for an ambulance.
Emergency services rushed to Peacehaven, East Sussex, at around 4am this morning after the 21-year-old dialled 999.
The woman, who was carrying a torch, was found at the bottom of a cliff with serious injuries, police said.
Sussex Police say the fall is not believed to have involved anyone else but an investigation has been launched into the "unexplained" incident.
A force spokesman said: "Officers are seeking information from residents and drivers in Peacehaven after a woman was found seriously injured at the base of seafront cliffs.
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"At 4am today emergency services were called to the seafront at the bottom of Cornwall Avenue, Peacehaven, after a 21-year-old local woman had been found lying there with serious injuries.
"The woman was conscious and had phoned the ambulance service herself telling them she was there and had fallen.
"She was taken to the Royal Sussex County Hospital in Brighton.
"The incident is currently being treated as unexplained although not suspicious and an investigation is underway."
Officers are asking any possible witnesses to come forward, with the woman thought to have been walking since 2:30am.
DI Claire Gill, of the force, added: "From our inquiries so far it appears that the young woman had walked along nearby Arundel Road, then across the main A259 South Coast Road and into Cornwall Avenue, then falling from the top of the cliff, some 60 feet.
"At this time there is no evidence of any third party involvement but we are making inquiries in the area and would like to hear from anyone with information.
"We would particularly like residents in that area to check any dashcam or door bell footage for the period between 2.30am and 4am so that we can see if it contains anything that might help the investigation.
"The woman, who is white, has blonde hair and was wearing a black coat, blue jeans, and was carrying a torch.
"If you can help please contact us either by calling 101 or online, quoting Operation Middle."
