The Royal Navy may hold records about possible sea monsters but it does not collect them centrally, it has been revealed.
Sailors can note unusual sightings on the ocean waves in their ship's logs, the Navy said.
But they are not required to do so and none of the information is assembled in a central archive devoted to sea monsters.
Any sightings of strange marine animals reported to the Navy by the public are passed on to the UK Hydrographic Office, which provides charts and other navigational services for mariners.
Details of the Navy's policy on giant creatures of the deep emerged in response to a Freedom of Information (FoI) request.
A marine biologist inquired whether the Ministry of Defence held records about "abnormally large or dangerous sea monsters hundreds of metres under the sea" that had not been revealed to the public.
In reply an official wrote: "The RN (Royal Navy), and MoD in general, does not maintain any form of central repository of information purely devoted to sea monsters.
"Personnel might be inclined to record unusual sightings in ship's logs but there is, as far as we know, no actual requirement for them to do so, and it would be beyond the resource constraints of an FoI request to check every line of every RN log book for any such references since 2005.
"However, the RN does invite people to report sightings of marine mammals, and it's possible this could include unusual sightings.
"These are forwarded to the UK Hydrographic Office at Taunton."