[norton]

"Little did we expect that, by using a lower frequency radar, we would find chambers, vaults..."

As I remember;
low frequencies pass through both water and earth - used them to communicate to subs and silos.
does not seem TOO difficult to figure out they'd be useful to spot underground crypts as well...?

[blam]

"low frequencies pass through both water and earth - used them to communicate to subs and silos."

Yup. there was/is one transmission antenne that is 2500 miles long. (part of it consists of bedrock)