The building, designed by John Bryce, is known as the Facade. It was originally built as the entrance to catacombs, but these were never built. So the space behind the doors only extends as far as the rock face, around two or three metres. It's where they store the big lawnmowers, as well as any of the bronzes or statuary that is are danger of falling off the back of a hearse, if you get my meaning.
Thanks, Sharon, for the "authority".