The estate at Witley Park in Britain has been a private home and a public facility at different times. What is visible above ground is nice enough, but the secret underground and underwater construction is a treasure. Deep passages lead to the rumored "ballroom under the lake", which, as it turns out, was originally built as a billiard room, but it wasn't the only glassed-in room. Guests can watch fish swim around them -or they could at one time or another. Link -via Metafilter
Also see: more pictures at Flickr. http://www.flickr.com/photos/cybergibbons/sets/72057594107271287/with/128621401/
Chamber of Secrets?