Omegaville archeological research site / mgabrys' underground (weekly) challenge submission
Rest of flickr set peek at : http://www.flickr.com/photos/87457200@N00/sets/72157629196076400/
Located near challenge spawn - at the edge of the water - look for the 2 surface ports (entrance and exit).
Story from walking tour : "Why is a 190 year old site still radioactive"? Be sure to stay on the paths (not that it's easy to fly down there - but if you do - you'll miss the fun).