Geograph - photograph every grid square

When I took this picture I thought that this mysterious looking structure had something to do with drainage. But thanks to Nigel Searle pointing me in the right direction, I now know better.

The truth appears to be revealed in a posting on the Canalworld discussion forum (Link.

It is a ventilation shaft for a tunnel containing a 24" gas main. The tunnel entrance was alongside the former Maid Boats boatyard at Ferry Yacht Station on the opposite side of the river. Once one entered into the building there was a long shaft which dropped to the floor of the tunnel, access being by a metal 'fire escape'-style staircase. From underneath the river - one could walk alongside the pipe - it was possible to hear the craft engines as they motored overhead.

The National Archive holds a record entitled 'The Barge Walk: tow path; licence for ventilating shaft and gas main under River Thames' (Link.