Where is all the underground WTC video post collapse?

I recall during those dark days that a group of structural engineers as well as rescue personel made an attempt to see how far into the the WTC structure they could go using underground pathways / part of the PATH subway as well. Snipets were played on national TV during the 24/7 coverage we had at the time.

However, since then I have never hear, nor ever seen any real discussion or the footage of this anywhere. Can anybody shed some light?