Perhaps we can create an archive of websites devoted to or related to Hollow Earth theory. Some are well known to the regulars here of course.
Hollow Earth insider by Dennis Crenshaw
Jim Nichols more UFO than Hollow Earth
Hollow Earth blog with great articles
Hollow Earth blogger exploring the scientific method.
Archive of Hollow Earth material from Bibliotecapleyades
http://www.angelfire.com/ut/branton/redbook_geoconcaviticsphere.html