[Emile] Baraize [in the 1920s] also paved with cement a deep hole on the top of the Sphinx's head. The hole measures approximately 5 feet square and nearly 6 feet deep. An iron trap door was fitted to the mouth of the hole. It has been theorized that the hole, began as a means for affixing a headdress to the sphinx in the manner of the New Kingdom, was later deepened in search of hidden chambers.
You can read more about the various holes, passages or tunnels at the Sphinx, here:
I'm also of the opinion that the Sphinx was originally Anubis and that the leonine recarve came later.
This may be of interest:
Edited to correct a typo.
Edited 1 time(s). Last edit at 16-Mar-19 07:11 by eyeofhorus33.