It is the surviving relic of a world war that thousands of people in Belfast drive or walk past every day without even noticing.
For hidden in plain sight in the wall that protected the former Donegall Pass police station from bomb attacks during the Troubles is a throwback to a defence from the Second World War.
A closer inspection of the perimeter fortifications at the Pakenham Street junction reveals front and side walls from an air raid shelter that was there at the height of the war with Germany.
"It's quite amazing when you study it and realise what it was," says Emma McBride, an archaeologist in the Historic Environment Division of the Department for Communities (DfC), who is now at the heart of the quest to find more physical reminders of the wars in Northern Ireland before they disappear for ever.
And the DfC team tasked with pinpointing the structures for an in-depth survey has predicted that they will discover thousands of them.
The remains of many lookout towers, trenches, gun posts, flying boat and naval bases, as well as prisoner-of-war camps, have already been recorded.
But the surveyors are now looking to the public to direct them to other less obvious links to the past.
The two-year 'audit' by the DfC is aimed at establishing a Defence Heritage Record (DHR) of all of the wartime defence structures.
The first phase of the work on the ground by Dr James O'Neill is focused on the 19 as yet un-surveyed airfields around Northern Ireland which played crucial roles during WW2 and which now have very different uses as prisons and race tracks.
Other buildings have been turned into homes for people or cattle and others are used by commercial firms.
James O'Neill, who is an expert in defence heritage, says even though a lot of the structures are marked on maps in RAF archives, he is still on a voyage of discovery.
He adds: "We were at the former RAF Bishops Court airfield in Co Down recently and there were radar buildings that were not showing up on maps and some structures that were added during the Troubles. It was a real buzz."
James acknowledges that his work can be a race against time.
He recently tweeted that on a visit to the former RAF Kirkistown, one of Northern Ireland's three dedicated fighter stations during WW2, he arrived to find one building being demolished.
Another tweet revealed that at Maghaberry, now home to our biggest jail, James was surprised to come across a hangar with a gantry crane from 1941 in perfect condition.
James says that the prison takes up only a tiny part of the old RAF airfield - just like the Maze prison at the former Long Kesh airfield.
One of the most important airfields was at Ballyhalbert which provided protection from Luftwaffe raids on Belfast.
Construction started in late 1940 and involved knocking down a historic windmill.
The completed airfield, which was used by airmen from around the world, featured three tarmac runways, two Bellman hangars and 13 temporary Blister hangars.
Parts of the old runways, air raid shelters and a firing range are still there and a group of aviation enthusiasts are trying to raise funds to restore what's left of the control tower at Ballyhalbert. It was abandoned in 1947 and sold to developers 13 years later for caravan parks, housing and commercial interests.
The seeds for the defence heritage project were sewn in 1997 as part of the Defence of Britain initiative which was launched to identity the UK's wartime landscape that was being lost through the ravages of time.
Emma said: "A small group of volunteers were involved in the scheme and they identified about 700 sites within Northern Ireland but only about of half of them were ever visited for surveys. But now we have launched the £90,000 phase two of the project to actually record the sites, many of which are on farms and other land where the owners perhaps don't see the heritage value in them."
The DfC's mission statement says the province's defence heritage represents a significant period in its history and they want to work with councils, heritage groups and community organisations to conserve it.
They cite Grey Point Fort in Crawfordsburn Country Park as a prime example of an historic monument which is now in state care, after having been used to protect Belfast Lough from potential naval attacks.
Emma said: "We won't be able to save all of the structures that are identified, but at least there will be a record of their locations."
Emma is excited by the past. She said: "If you go over Craigantlet hill for example you can see what's left of old buildings where late-night decoys were created to simulate burning towns and cities in a bid to divert German bombers from their intended targets so they would drop their ordnance over the countryside."
Nearby in the Stormont estate, where Parliament Buildings were covered with pitch and manure to protect them from German bombers, the anchor points for barrage balloons which were also shields can still be clearly seen, while a bomb crater from the Blitz is fenced off near the Upper Newtownards Road entrance.
Several buildings dating from the war have been used in more recent times as bases for cycling clubs and scout troops.
At the former Ballykinlar Army camp an old WW1 chapel and cinema was in operation until three years ago.
Emma McBride says the DfC's eventual plan is to upload all the information they collate from the surveys onto the internet to "provide present and future generations with an understanding of important physical features and an explanation of their locations during the wars".