By their very nature, hidden rooms are kept secret and, more often than not, the reason for such circumstances are rather nightmarish.
YouTube user Dark5 has compiled a haunting collection of notorious hidden rooms with nefarious purposes that shocked the world when they were revealed.
Perhaps the most unsettling case detailed in the video is that of a secret basement found at a children's home in the Channel Islands.
Ostensibly a place where neglected children could be cared for by the state, the Haut de la Garenne seemingly proved to be anything but that.
After the alleged discovery of bone fragments around the facilities, authorities investigated the building and found a heretofore-unknown room in the basement.
It is believed that the spot, a twelve-foot-deep pit, was where misbehaving children were sent as a form of solitary confinement and, no doubt, abuse by workers who tortured the unfortunate tots.
And, in an absolutely chilling detail found by investigators, a wooden pole in the basement bears the message, "I've been bad for years and years."
Other horrific hidden rooms discussed in the video are the torture chamber of infamous serial killer H.H. Holmes at his hotel in Chicago and the underground home-made prison fashioned by the monstrous Yosef Fritzl in Austria.