Amazing House

This house is called "The Subterranean Mansion" and it's located in England. It's on sale for £2,000,000.The house is not that big. There are several bedrooms, WC's, etc. But there is something that makes it one of the most daring innovative houses ever built. Do you see this round hole in the bedroom wall? Any ideas what it is for? Just take a look at the second photo.