What was that then? A way down to the landing stage or something?



As for the tunnel exposed in this picture, on the right...



...that's a storm drain running parallel to the inlet, from an overflow weir by the Liver Building. At the weir, a 2.5m diameter concrete combined (foul and surface water) sewer runs past in a chamber.

The idea is that if there was a lot of flooding and the sewer filled up, the waste would overflow into the brick built (Victorian I assume) storm drain and go into the river rather than backing up in the streets/houses. Smaller surface water drains are also connected to the storm drain.