To ensure the structure beneath the Leigh Woods tower was strong enough to support its weight and transfer the load of the bridge into the rock face, Brunel planned a cost effective network of vaulted chambers.
As there was no access to the completed abutments, many people assumed that they were solid. In 2002 builder Ray Brown discovered the chambers in Leigh Woods when replacing paving.
‘We found the first one by chance really, we were going over it and a little hole appeared because the wood that they’d fitted to cover the shaft up had rotted. I poked a rod down there and they found one big void.”
Guy Barrett was one of the first people to go inside, lowered down a shaft from the surface with no idea of what he might find below:
"When they took away the railway sleepers they found a shaft just disappearing in to the blackness. They shone a torch down and couldn't see the bottom. No-one knew what was inside the abutment - they didn't know if it was solid or hollow. I think some cores had been drilled earlier? So it was arranged for us to set up the equipment and go down and have a look and take photographs.
The first shaft that we went down was on the upstream side of the abutment of the pier and I was lowered down into it - went down through through the hole and after about 20 feet or so there was a tunnel going out horizontally either side of me - each of those tunnels probably about a size that I could get through: just!
There were stalactites, or the calcite deposit from the lime mortar that had dripped down through the bricks and then they carried on lowering me down. I went down probably another 40 feet or so and there was another set of tunnels leading horizontally either side of me, exactly the same thing. And then went down eventually to the bottom, was probably another 20 feet beyond that just down to rubble in the bottom of this brick shaft.
It opened out into a chamber 30-40 feet high and wide. Once we'd gone inside that we got to the bottom of that and found all sorts of tunnels leading off in a sort of honeycomb. The real sense of excitement was going somewhere that nobody else has been since the Victorians."
In Summer 2016, Bill Harvey, Hamish Harvey and Dave Anderson made their way to the lower levels as part of a maintenance inspection.