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He and a friend had found out about a 700-year-old network known as the Caynton Caves on the internet, and decided to go and find out more.
In the video description, he explained: "We found secret hidden underground Knights Templar caves down a rabbit hole while exploring in the Shropshire woods! Join me on this incredible explore as see how far the rabbit hole goes!"
After finding the entrance beneath a tree, Brendan and his pal clambered through the hole into the cave, discovering an eerie underground complex covered in litter and graffiti.
Brendan said he felt 'dubious' about going down, but was surprised by what he found - despite feeling 'claustrophobic', adding that the area 'actually looks really creepy'.
While he believes the caves could have been built by the Knights Templar, a Catholic military order founded in 1118 in Jerusalem, he doesn't share any proof of this in the video.
What's more, according to Historic England, the 'grotto' was actually 'probably late C18 or early C19', although it adds that it is ultimately 'un-datable'.
"Hollowed out of rock in side of disused quarry, approached by very small unmarked entrance," it says of the cave complex.
"Plan, apparently irregular ambulatories opening into inner sanctuaries. Columns left by excavation as cylinders with bases and capitals (and occasional anuulets); neo-Norman decoration to bays between columns, one neo-Norman doorway with beak-heads and roll moulding; decorative quatrefoils and designs abound."
At one point, the two explorers found a passport-sized photo of a man, with Brendan's friend saying the site is supposedly haunted due to Satanic and black magic rituals that were once carried out there.
Another moment saw Brendan's light stop working, despite being loaded with four fully charged battery packs.
The pair began to feel concerned about the safety of the cave structure when they found a collapsed wall, and eventually decided to leave.
Brendan said: "I didn't think I was claustrophobic, but I was thankful to get out."
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