Urban explorers are often discovering things that take the rest of us by surprise, but I think this would shock even the most experienced of adventurers.
The ExploreMore group had come across an underground nuclear bunker built during the Cold War.
So far, so normal right?
Well, they weren't the only ones there.
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It turns out that a music rave was going on down there, too.
House music was pumping out of the speakers inside the Brislington War Room, a Grade II-listed site in Bristol.
Fortunately, the revellers were the friendly kind as they invited in the explorers in to join them.
Speaking to Metro, one of the explorers, Dom, said: "They allowed us entry to the site with their own key to the gate,
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"Walking through the site they told us they hold regular events all over but it’s all hush hush, invite only.
"We arrived at a huge metal door and when it opened smoke plumed out and we could hear the pounding from the speakers.
"They were all really friendly and didn’t mind us taking photos.
"The bunker was in pretty good condition as they had cleared rooms for the rave, using squeegees to provide safe and dry walkways."
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According to Historic England, the facility - also known as the Bristol War Room - was built in 1953.
It was constructed to ‘co-ordinate civil defence in the event of an atomic attack’.
The two-storey unit is shielded by reinforced concrete and contains items which have not been touched for decades.
When used for raves, it has been adorned with fairy lights which hang above the desks originally used for switchboard or teleprinter operators.
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Dom said: "Original features were left alone like the line of tables where call handlers and telecoms would have been positioned and the maps still hung on the walls.
"They had fairy lights hung around the bunker for a more trance-like vibe but they had managed to get the mains working again too so all the original lights were on as well."
Historic England describes the two-storey, semi-sunken facility as ‘a rare survival of a purpose built war room’.
In the event of an attack, the regional government occupants of the bunker would have been responsible for Home Defence Region 7.
This included the South West, including counties like Cornwall, Devon, Gloucestershire, Somerset and Wiltshire.
Ben Thompson