It's like something from a children's mystery book - a hidden wooden door which leads you down stone steps and into a boat for a short underground row to two large doors - and then, into the cavernous ballroom hidden under the lake; green algae coating the tiny windows speckling the dome.
According to those who've dug into Witley Park's past, few know of this sub-water billiards room, and even fewer have actually been allowed inside. Constructed in 1889 by Whitacker Wright, who had done very well in the mining business and built himself a 32-bedroom country manor to fit his extravagant, if slightly-eccentric, tastes.
It was initially thought that the Witley Park room was a ballroom, but apparently it was built as a billiards room, with a conservatory room for fish-watching. Unfortunately Wright didn't have long to enjoy his sub-water games room, nor piles of coins, and killed himself by ingesting cyanide at the Royal Courts of Justice, having minutes previously been found guilty for fraud.
The next owner didn't have much more luck with his wealth - Irish shipbuilder Lord Pirrie bought Witley Park in 1909, and while he had a close brush with death after he narrowly avoided travelling on his finest creation, the Titanic, due to illness, eventually pneumonia finished him off in 1924 while sailing around Cuba.
Now owned by a "communications mogul" by the name of Gary Steele, the mansion burned down in 1952 but the billiards room remains shrouded in underwater secrecy (and algae.) [Sabotage Times and Talkurbex via Metafilter and Neatorama]