Rabbi's Tunnel...it runs along the western wall of the Temple Mount under the ground.  Since the Temple Mount was built on a slanted hill, leveled off at the top, you can see Herodian stones set right on the bed rock...which is one of the reasons it is in such great shape today.  This idea of going down to the bed rock was apparently a new concept at the time. (see picture below)  The stones down here are in seemingly "mint" condition, especially considering their age.

This picture shows a Herodian stone (note the dressed edge) set into the bed rock.  Amazing engineering for the day...such tight fit and it obviously has endured the test of time.