FIREFIGHTERS were called to Sunderland Royal Hospital to deal with a two metre-deep flood.
Crews from across the city went to the scene after the water triggered the main fire alarm.
Specialist fire engines were called in to help pump the rising water out of the building yesterday.
It is believed ducts below the hospital burst, causing a network of tunnels to flood.
The water did not affect the hospital and no patients had to be evacuated.
Firefighters remained at the scene for more than two-and-a-half hours clearing the water.
John Allison, Tyne and Wear Fire and Rescue Service group manager, said: “A large volume of water, about two metres deep, was in the ducts below the hospital.
“We had to wait for the water levels to go down before it could be established what the cause was.”
A spokeswoman for Sunderland Royal Hospital said: “Water has been found in a section of the underground access ducts on the Royal Hospital site.
“The fire brigade were called to pump out the water and there has been no disruption or loss of services to any wards or departments.
“We suspect that a pump located in the duct which routinely takes out excess water has malfunctioned although this is yet unconfirmed.
“Our priority is to pump out the water and to check all service areas which will be carried out today.
“A full investigation will be undertaken to determine the exact cause of the flood and to ensure that lessons are learned.”
Fire officers left the scene at about 2pm yesterday.