Two weeks ago I published a picture of one of the two cages that were located either side of Portscreek protecting the two shafts that dropped down to a tunnel which ran under the creek.
Thanks to Ian Limb of Portsmouth Water Company I am able to tell you more.
From early in the 19th century until the Second World War the only route on to Portsea Island for mains water supplies was through the Hilsea tunnel.
In 1901 the Admiralty asked for a substantial increase in water supply. A 20in pipe was to be laid from Farlington to the Dockyard for its exclusive use.
It was to have been laid at a normal depth, just beneath the low tide mud under the creek at Hilsea. The Admiralty had plans to deepen the creek to enable shipping to pass between Portsmouth and Langstone Harbours.
In 1903/04 a deep tunnel was constructed under the creek to supply the whole of Portsea Island with water from the ‘mainland’. (It still amazes me how many people do not know that most of Portsmouth is actually an island).
The tunnel is 600ft long and the top is about 36ft below sea level at low tide.
When first built it contained four trunk mains one of which was the 20in dockyard main. The Admiralty’s scheme to deepen the creek never happened.
The tunnel remains in service today albeit with renewed mains laid through it.
In the black and white photograph we see the massive pipe that still lies under the creek. The tunnel section is 12ft 6in in diameter and the shaft section 15ft in diameter.
You can see why the two cages were built over the opening to the two shafts.