Underground History

Belsize Park deep level shelter site visit

Belsize Park's deep level shelter is an excellent example of what could be found in a World War 2 deep level shelter on the London Underground as many of the fittings and plant have not been removed. The shelter is accessible via one of the original pill box buildings on the junction of Havestock Hill and Downside Crescent. These buildings are quite distinctive and each shelter would have had two. Most still survive today and can be seen a short distance from the Underground station entrance.

Surface pill box building

Belsize Park deep level shelter surface building

After a short wait for everyone in our group to arrive, it was time to enter the lift, and travel down to shelter level. There were seven of us in total and since the lift was quite small and could only take four people at a time, I joined the first group and entered the lift. At least this was better than plodding down the rather steep looking spiral staircase...

Lift - surface level Lift - shelter level

The lift and spiral staircase

We reached the bottom and waited as the slow lift went to the surface and collected the other three. After a while we could hear them talking down the lift shaft asking where we were! It became apparent that we had traveled to the bottom deck of the shelter and they were on the top. Each shelter had two layers where the circular tunnel had been split horizontally.

No problem. We quickly started climbing the spiral staircase only to discover that we mysteriously climbed past the top deck without passing an entrance - it turned out that there are actually two spiral staircases in the shaft forming a double helix - one staircase for the top deck and one for the bottom. In the end, we climbed back down to the lift and used it to go up one floor to the top deck! This also explained why the staircase was so steep.

Outside the lift

Just outside the lift.

On leaving the lift area, it became immediately apparent that although this place had long since finished being used as a shelter, the location had found a new use. It seemed that it had become the subterranean hell that all cardboard boxes go to when they die. There were boxes everywhere! The entire complex is today being used to store paper documents in literally thousands of cardboard boxes. It seems ironic that in the digital age we're living, the entire contents of all the documents stored in the over 800 metres of tunnels of this complex could probably be stored on a single hard drive! The fact is though that official paper documents are still required in many cases for legal reasons.

There was an interesting feature with this interconnection tunnel. Looking at the above photograph, it can be seen that the tunnel occupies more than 180 degrees of tunnel space. This is true of the tunnel in the bottom deck as well. This is effectively two tunnels merged into a single figure 8 tunnel - something that hadn't been attempted anywhere else on the Underground. This was only done on the interconnection tunnels - the main shelter tunnels are circular.

Immediately to our right, having left the lift shaft area was a doorway marked Danger - Keep Out. This was the door to the shelter's plant room.

Plant room door

Plant room entrance

The plant room consisted of equipment to power the lift and the ventilation fans. The plant consisted of a transformer, a mercury arc rectifier and some equipment to control the vent fan's intensity. Although the ventilation system has long since been decommissioned, the plant was still in use today controlling the lifts, though the mercury rectifier seemed to be powered down (more about rectifiers later).

Plant room door
Plant room door Plant room door

The Plant Room. The second image shows the ventilation controls, the third shows the non-functional mercury arc rectifier.

Immediately outside the plant room, the large ventilation fan could be seen. This fan led out to a separate ventilation shaft that ran to the surface. The fan sucked air out of the complex causing fresh air to be drawn in down the spiral staircases. I imagine it would be quite noisy not to mention a bit draughty standing here when the fan was running at full speed...

Ventillation fan

Ventilation fan

Having inspected the plant room it was time to move on. We were still not in the main shelter tunnel run - there was a short walkway before we reached there. Before arriving at the tunnels, we passed the area that would have originally been used as toilets for the shelter. They were no longer equipped with toilet facilities, but now contained... more boxes!

The entrances to the toilets on the main access tunnel

Entrances to the toilets, off the main access tunnel

Having now reached the main tunnel, area, a small passage could be seen across the two main tunnels leading in the opposite direction to the one we were entering. This would have been the medical station for the shelter. We turned left and headed south down the main shelter tunnels.

Shelter, now used for document storage

The shelter tunnels, now full of documents

After a long walk down the tunnel, we arrived at the second spiral staircase which lead to another pill box building on the surface. Theoretically, the lift still worked, but was no longer in regular use since the surface building was no longer in use. On the surface however was a similar pill box structure to the one we had entered earlier.

The facilities here mirrored those of the ones we had seen previously. There was a short spur tunnel which would have been used as a medical station, and another tunnel which ran to the staircase and a plant room.

This plant room however differed the previous plant room in one important way. As I walked down the corridor into the plant room, there was a distinct ozone smell to the air. Further, as I entered the door, I saw the most eerie sight I've ever seen while investigating tunnels. In one corner of the pitch dark plant room room was a glowing, shimmering purple ghost... who from a distance appeared to be making a rather rude gesture!!!


What I was in fact seeing was an original 1941 mercury arc rectifier, which was still in use today, converting mains alternating current to the direct current required by the lift machinery. A mercury arc rectifier consists of a pool of mercury in a vacuum tube. When powered, it glows with an eerie purple luminescence and on closer inspection, what looks like a small, bright bluish flame can be seen dancing on the surface of the mercury pool. Traces could be seen on the glass surface where mercury had condensed and trickled down the inner surface of the bulb.

Since the glass tube has had all the air removed forming a near vacuum, the mercury forms a vapour within the tube. The rectifier works because a plasma arc between a pool of mercury and a metal anode only allows current to pass in one direction. To find out more about mercury arc rectifiers see here.

Mercury Arc Rectifier

Mercury Arc Rectifier

Looking at the rectifier was mesmerising, but there was more to see (besides, they emit a lot of ultraviolet light so looking for too long is dangerous). Just outside the plant room, next to the large fan was a sump, which allowed water to collect to the deepest part of the complex and that was where we were to go next.

Bottom of the ladder

After climbing down a short iron ladder, a small tunnel snaked to the sump's actual location. A small rusty trapdoor could be seen in the middle of a circular enclosure (which turned out to be the bottom of the ventilation shaft). To one side, some tanks could be seen - these were the high pressure tanks used to pump sewage to a surface sewer. A short, but tricky climb up another iron ladder through the roof of this structure led to the other side of the ventilation fan. It was possible to look directly up the ventilation shaft at the top of the ladder.

The bottom of the vent shaft The small rusty trapdoor to the sump

The bottom of the ventilation shaft. The tunnel leading off is the access tunnel we'd just come through. The iron ladder up to the fan can be seen to the left of the first picture. The second shows the trap door over the sump.

High pressure waste ejector High pressure waste ejector

High pressure pumping equipment to eject waste to a surface sewer.

The other side of the ventilation fan The small entrance at the top of the iron lader

Inside the fan chamber.

Coming back out of the sump area, we walked back out into the main tunnels all the way to the southern end. At times, trains on the northern line could clearly be heard running above us. In some places, the World War 2 bunks were still visible - but now they were being used as shelving, rather than bedding.

Bunks being used as shelves Bunks being used as shelves
A bunk is used as a doorway

Bunk beds being used as shelving and as a door

On our way back, we came across a staircase that led upwards from the shafts. This was the original link tunnel to the main Underground station platforms. Some original gas filters could be seen near a doorway and many bunks which had been removed from the main tunnels had been stored in the small walkway. The actual entrance to the station had been bricked over, but the remains of an original telephone was still there on the wall. The shelter would have had its own telephone system and local exchange, similar to the one seen in Down Street.

Stairway to Belsize Park tube station Original gas filters

Staircase to Belsize Park underground station, and the original gas filters that would have been used during a gas attack.

Cross-passageway with some removed bunks Bricked off access to tube station

A passageway containing some junked bunks and the bricked off access to Belsize Park Underground station

The remains of the telephone

The remains of a telephone at that location

And that was about all there was to see. After taking the lift to the surface, we inspected the lift machinery room - much of the original lift gear has since been replaced. Then, outside we looked at where the ventilation shaft came to the surface. This shaft was also where heavy plant would be hoisted down. Unusually, the vent shaft had a 30ft brick chimney. Most of the shelters were equipped with temporary chimneys which have since been removed.

The bottom of the vent shaft's chimney The top of the vent shaft

The ventilation shaft for the deep level shelter

Thanks to Nick Catford & Subterrania Britannica for making this visit possible.

Last modified: March 20th 2004

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