Two pals decided to try their hand at photography at this long-derelict address in Cologne, Germany.
Haus Fühlingen was used by the Nazis in World War Two to sleep forced labourers and has a long, dark history.
And now that story has a spooky new chapter, because these urban explorers discovered a boiler caked in blood in the basement.
“If something happened, it would have been pretty hard for anyone to find us”
Simon told Reddit how he and a friend stumbled across the boiler while exploring using the lights on their phones.
Both had a growing interest in photography and were after an interesting location to take some pictures.
They ended up at Haus Fühlingen, where Simon’s camera flash revealed the bloodied basement boiler.
Horrified, the pair made a quick exit – leaving the grizzly scene unexplained to this day.
“It was pretty stupid to just go down there without thinking about it,” said Simon.
“I think there was just one friend of mine who knew where we were.
“If something had happened, it would have been pretty hard for anyone to find us.”
Koeln.de, a website run by the authorities in Cologne reveals the bloody past of Haus Fühlingen.
During the war, a Polish forced labourer called Edward M fell in love with the landowner’s daughter, the site says.
So enraged was the underage girl’s father that he had the Nazi secret police, the Gestapo, apprehend and hang the boy nearby.
Edward, aged 19, had never approached the girl and his spirit is now said to wander the mansion still searching for her.
After the war, former Nazi judge Gerhard van K lived there, until he took his own life on New Year’s Eve 1962.
In the years after his demise, legends persisted of a mystery woman living alone in the crumbling ruin.
This woman, it transpired, was Gerhard’s widow Alice, who was moved to a nursing home where she died in 2000.
A body was found on the first floor of the property in 2007, according to the Kölner Express newspaper – an apparent suicide.
Reddit fans pointed out that the blood on the boiler would, with time, turn brown – meaning it was either fake or freshly deposited.