Vietnam Cave

I read in a childrens magazine recently that the longest underground river is in Vietnam's Phong Nha caves. It's listed as 40 miles long. Anyone familiar with this? I couldn't find anything related to diving there. How does the distance compare to what was recently discovered in Mexico?

You have to be careful here with the definition of "underground river". It is possible to have a stream/river in a "dry" cave, which is different from what is in most of Mexico, and Florida.

It is a world heritage site:

http://whc.unesco.org/pg.cfm?cid=31&id_site=951