Hi all was up walking round a old disused air base with my wife and kids when we came across what looked like a large man hole cover then we spotted a air vent coming out the ground about a foot away from the cover! the base is now a business park but dose still have a lot of the original buildings there! Ive added a few pictures of the man hole cove and air vent! it did have a bar with 2 padlocks on it for years but some one has cut them off| any one have any ideas what it could be?