The explorer – who remains anonymous but uses the name UrbexRotten – stumbled across the hole in the ground with his friend in Dover, Kent.
Nicknaming their find “The Asbestos Hole of Death”, the 36-year-old decided to abseil down a rope fastened to a 4×4 piece of timber.
Expecting the descent to only be 12-foot, the pair were astonished when they rappelled 88-foot down into a dressing station apparently used in both World Wars.
Images of the dressing station show the bunker left seemingly untouched for decades, with various paraphernalia lying around.
Oil cans, a chamber pot and even old shoes can be seen lying around the abandoned station.
An image shows the date 31 December 1940 written on one of the walls, seemingly signed by a member of the 172nd Tunnelling Company.
A branch of the Royal Engineers created by the British army in WW1, the company’s purpose was to dig and maintain trenches, subways and underground chambers such as dressing stations.
Believing the station to originally date back to the 1800s, UrbexRotten shared their findings on Facebook last week.
He said: “Me and my friend found a hole in the floor. It was a rainy Sunday and the hole just looked too deep and too dangerous to venture down.
“Two or three weeks later we went back with a solid piece of 4×4 timber and some rope.
“Expecting the abseil to be around 10 to 12-feet were shocked that the descent was actually 27 metres.
“Do not attempt. This discovery was a dressing station.
“Dressing stations were located in abandoned buildings, dug-outs or bunkers to protect from shelling.
“Sometimes they had to use tents. Each station would have 10 medical officers, medical orderlies and stretcher bearers.
“From 1925 nurses were used in the chain of casualty evacuation.
“This dressing station served the two gun sites housing ‘Winnie’ west of St Margaret’s and ‘Pooh’ east of St Margaret’s.
“Six different entrances with steps down lead to a main tunnel 276-feet long connected to a parallel tunnel. Construction is mainly unsupported chalk.”
The only light visible in the images comes from UrbexRotten’s flashlight, which makes for an eerie feeling in the bunker.
The photos have gained impressed reactions from social media users, who were in awe at the find.
KHUrbanX said: “Oh wow, I thought this place was long gone.”
Clare Smith said: “Wow that’s incredible. Thank you so much for sharing.”
Laura Smith said: “This is” alongside an emoji of the OK hand sign.
Liam Phillip Clark said: “Cool find mate, would say myself, wouldn’t go in there, just the size of the entrance.”
Speaking today, UrbexRotten said: “The dressing station was rather scary. We explored it back in September, and didn’t expect the abseil down to be that long.
“The climb up was scarier as claustrophobia was setting in. I vowed never to go back down.
“It was fascinating down there as there was all kinds of old stuff from the past, like old cans, pots and shoes.
“I’ve done a lot of travelling and whilst abroad I used to visit abandoned places like Chernobyl.
“I was bored in the first lockdown so I thought I’d do it in the UK.
“Old war tunnels, air raid shelters and bunkers are my favourite but I’ll do anything abandoned basically.
“I think this one actually originates from the 1840s at least, judging by one of the carvings on the wall.
“You’ll find loads of these Napoleonic defences around coastal counties though. They also served a purpose in both World Wars.
“You can tell when you visit some locations that they altered them for WWII, like if they’ve been reinforced with brickwork and there’s evidence of electric lights that wouldn’t have been around in the mid 1800s.
“The war stuff isn’t that hard to research online, you just need to know names of old forts etc and build your knowledge from there basically
“I’ve been doing it since 1st lockdown and hit over a 100 locations so far – and know of a lot more to do in the coming year.”