For a city that doesn't have an underground network, Bristol has more than its fair share of unusual subterranean nooks and crannies that prove tempting for urban explorers and history buffs alike.
From tunnels underneath Castle Park that date back to the 12th century when Bristol was the centre for wine imports, to a below-ground Victorian street in Lawrence Hill, to an underground funicular which later served as both a World War Two bomb shelter and secret transmission centre for the BBC, beneath its polite, gentrified surface, the city is brimming with subsurface secrets.
Attempting to access most of these hidden hideouts will land you in a fair bit of trouble, as they are strictly off-limits. However, for fans of dark caverns and caves, there is a place in Bristol that's not only safe and legal to access but also considered one of the city's most underrated tourist attractions.
Read More: Inside Bristol's secret hidden tunnels from the Middle Ages under Castle Park
Unsurprisingly, the Clifton Suspension Bridge vaults are part of what is arguably the city's most popular draw - Isambard Kingdom Brunel's bridge that spectacularly spans the Avon Gorge. Building began in 1831 but it was not completed until 1864, after Brunel's death, using designs by William Henry Barlow and John Hawkshaw. All drawings related to the early phase of the bridge's construction were lost, most likely in 1843 when works halted for several years because of a lack of funds.
At this point, the two towers were already built, with the Clifton side perched primarily on a large outcrop of rock while the Leigh Woods side sat on a large brick abutment that rises from the Gorge. The massive 33.5m (110ft) high abutment was the first part of the bridge to be constructed between 1836 and 1840, and up until the 21st century, its structure remained a mystery.
Engineers had suspected that it was hollow, but borehole investigations carried out by Bristol University in 1969 managed only to find a solid structure. In 2002 twelve vaulted chambers up to 36 feet (11m) deep were discovered accidentally by contractor Ray Brown during works to resurface the abutments.
The vaults are spread over two levels, seven on the first-floor level and five on the basement level. Both stories feature a long central 'hall' vault, surrounded by four smaller but tall chambers known as the corner vaults. On the first floor level, two further vaults can be reached through a corridor from the central chamber, and a triangular space lies beneath where the road joins the top of the sloping rock face.
Going underground
Open to the public since 2017, the Clifton Suspension Bridge Trust offer three different tours, including a visit to two chambers between spring and autumn. For those wanting in-depth history, there's a two-hour hard hat tour priced at £20 per person or a one-hour version for £15.
Short on time but keen to experience the vaults, I opted for the £10 underground vault visit, a self-guided, roughly 45-minute visit to explore these unique underground chambers.
All the tours begin at the bridge visitor centre on the Leigh Woods side and require you to don a hard hat and high-visibility vest before heading past the coffee van, through a locked gate and down a path along the bridge.
The self-guided tour is escorted by two knowledgeable guides, and felt very safe. However, visitors are reminded of the risks involved and asked to sign a waiver before participating.
After skirting around the edge of the abutment, past the foundations of Brunel's 'suspended traveller' (a basket car hung from an iron bar 305m long and 3.8m thick that allowed the bridge builders to cross the Gorge), everyone has to pass down a vertical ladder before arriving at the doorway cut into the side of the abutment to allow access directly into Vault 4.
Marvelling at the thickness of the wall and the massive size of the bits used to drill into it, it took a while for my eyes to adjust to the darkness and make sense of my surroundings. The cavernous arched room resembled something from the TV show Knightmare, with small mysterious tunnels leading off in all directions.
A chunk of jutting rock from the Avon Gorge takes up much of the space, glistening with the damp and sporting short fat stalagmites. There's also a large pile of rubble thought to have been thrown in by workers building the tower above, though apparently, the only traces of humans found when the vaults were discovered were a few clay pipes and pickaxes.
The most striking feature is the long, thin ghostly white stalactites dangling from the ceiling, dripping and swaying ever so slightly. Known as straw stalactites, these bony-looking spindles are formed by rainwater leaking through the calcium carbonate structure.
Entering the nave-like vault five through a small tunnel, there was more opportunity to admire these stalactites, with some small chunks passed around and one even dropping from the ceiling before immediately disintegrating in slushpuppie-like powder. A wooden deck has been built in this room, making it easier to look around and allowing the space to host the occasional musical event.
To demonstrate the unusual acoustics of the network of chambers, the guides pop a balloon revealing the room's dramatic reverb. Both guides were very knowledgeable and happy to field questions, but if time and funds allow it, I suspect the hard hat tours will enable you to get more from your visit.
However, the short self-guided tour is ideal for families (children aged 7+ can take part) looking for an unusual excursion during the holidays.
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