The small hole, which is situated below Grève de Lecq barracks and above the main car park, appeared on Sunday and was reported to the parish’s honorary police and Jersey Fire and Rescue Service.
Antony Gibb, a historic building consultant and the vice-president of the National Trust for Jersey, said: ‘As the room is brick it’s probably pre-Second World War, so perhaps it has something to do with the barracks.’
He added that the chambers could possibly be part of a sewage system or even a store for munitions.
‘Without inspecting the space it is difficult to say,’ Mr Gibb said.
Grève de Lecq Barracks, a National Trust for Jersey property, is the only surviving barracks left on the north coast of the Island and still retains many original features, including two prison cells and the ablution block.
Martin Maguire, who was the duty officer for the fire service on Sunday, said: ‘The hole is no bigger than the size of a person and a crew member just looked down and saw there were two brick structures which were joined together.
‘We just had a look to make sure it was safe and the police have now covered the hole with a board.’
