A man named Collins followed up 19th C accounts. He will detail his findings in a book "Beneath the Pyramids" to be published in September.
Zhi Hawass says they do not exist.

On another Egyptian chat line I visit, there has been an extensive discussion concerning the finding of the caves.
In most opinions, mine included, it's all just a big hoopla to promote his new book!
There is absolutely no evidence of such caves, and no mention of them in accredited Egyptological writings.
I understand there is even photos published on his web site, alluding to be the caves. I looked at them, and they seemed very photoshopped to me, and several were mis-labeled as to where they were supposed to be!

Some of the passagways beneath the great pyramid are believed to be natural.