When Sebag was researching his Jerusalem: The Biography
I sent him this image (and a PhD in Hebrew to go with it).
I wondered if it was linked to the temple Hadrian built there, or to Julian the Apostate's briefly started Third Temple.
published photos from the 1920s and 30s which were titled along the lines of "Ancient entrance to Temple beneath el-Aksa."
These photos - look at the others they posted too - show the Roman staircase with Corinthian columns leading up to the Temple Mount. It was almost certainly built when Hadrian erected a temple on the site - that it turn was destroyed under Constantine, and its parts used to built the mosque above.
Until now Israel Daily Photo have been posting Library of Congress photos of the tunnels beneath the Temple Mount - but the great news is that the IAA just released hundreds more:
Eureka! Pictures Beneath the Temple Mount Now Online
I urge you to take the time to click through to their post, as they have a fabulous selection of photos taken after the 1927 earthquake hit Jerusalem, and before all the major reconstruction of the mosque took place.
This post is also worth reading for modern photos, plans, etc.