This is a very old article. One of the pages links to this page - http://www.nationalgeographic.com/nyunderground/docs/myth000... - which suggests using RealPlayer 3.0 and Shockwave 5.0.
Oddly they've taken out the (c) 1997 part, maybe to make it not seem as ancient? I suppose it's still reasonably relevant; not much of this basic information has changed. Heck, Water Tunnel #3 is still under construction.
> © 1996- National Geographic Society. All rights reserved.
So is it safe to guess it's from 1996 or later?
Still cool to see how engaging you can make a site using 10-15 year old technology. Content is king.