I just booked a tour of this fascinating site, newly opened for visits with a guide (only Saturdays and Sundays):
http://www.coopculture.it/en/events.cfm?id=268
You can read more about it and see photos here (but book on the CoopCulture site above):
http://www.tickitaly.com/galleries/domus-aurea-rome.php
I mention this because it is not in Rick's 2015 Rome book and many have not heard of it.