Just before midnight on the 9th September 1940, sailors up on deck of the cargo sailing ship, Seven Seas were watching an air raid by German bombers over the Charing Cross railway bridge when an explosion hit the river and a large wave struck the ship. A few moments later, a large fountain erupted out of the River Thames that lasted for about half a minute before dying away.

Eyewitnesses by the Hungerford Bridge reported a bomb impact in the area, and the report was confirmed by other people in the area about an hour later.

The bomb had evidently hit something under the Thames, but the Northern Line seemed too far away from the fountain, and anyway, although the anti-flood doors were closed at Embankment and Waterloo Stations, there were no reports from tube staff of any flooding in the tube tunnels.

However, the bomb had indeed smashed through the roof of the Northern Line tunnel – or more specifically, a long since abandoned loop of the Northern Line under the Thames that had been in use for just a few years before being sealed shut.

Although not that well known at the time (and less now), the London Transport Board were well aware of the existence of the tunnel under the Thames, and surveyors used some access tunnels to get into the area and inspected the bulkhead doors that had been welded into place in the abandoned tunnel.

The tube tunnel was completely flooded and water was starting to leak out through the bulkheads.

It helps to explain what a disused tube tunnel is doing here and why it was abandoned.

Lets jump back to 1907…

What was at the time called the Charing Cross, Euston and Hampstead Railway had opened, running from just underneath the front of Charing Cross railway station to Highgate. The original 1902 authorisation had included extending the line down to Embankment, but that extra bit wasn’t constructed.

Shortly after it opened, the owners realised that was a mistake, and they really needed to extend it their new line to link up with the Embankment Station owned by the Metropolitan District Railway so started petitioning Parliament for permission to extend the line.

Plans to drive a tunnel towards Embankment Station and then build two sidings to the West of the mainline railway were abandoned after the Metropole Hotel objected – fearing the shallow tunnels would damage its foundations.

A year later – in 1910 – they submitted modified plans to run a loop under the Thames to the East of the mainline station instead and this was finally accepted by Parliament in June 1911 as the London Electric Railway Act.

The plans called for a single track running from Charing Cross underground station, down towards the river where it ran in a loop and back up to what it today’s Embankment Station, where a single platform was constructed and then back up to Charing Cross again.

The original plan had also placed the new platform next to its equivalent at the Bakerloo Line, but the new plan put about 30 yards away, and a new deep level corridor was dug to link the two lines.

Construction started in October 1911 and the new station opened on 6 April 1914.

Remember that at this time, tunnels were still dug by hand, and as they were digging only a few feet below the river bed, the tunneling head was pressurised to stop water flooding the working area. Despite that, they were able to dig the tunnel section under the river in just five months (Nov 1912-Mar 1913).

The below image is of the later construction of the Kennington Loop, but is the same method as would have been used for the Embankment Loop.

Actually, it wasn’t just the tunnel that was dug by hand, a look at the accounts ledger showed payments for the use of a number of horse driven carts to deliver equipment to the construction sites.

In case you think this is fanciful or the line never actually opened, here is a rare photo of the inside of a train carriage from May 1920 – and if you zoom right in, you can make out the railway map – showing the loop at the end of the line at Embankment station.

However, less than 10 years after the loop tunnel was completed, permission was secured to extend the railway again – this time down to Kennington.

To enable the extension, two tunnels were driven under the Thames from the South Bank and a new Southbound platform constructed at Embankment station. The original terminus platform is still in use to this very day, as the Northbound platform of the Northern Line – which explains why that platform curves, when the Southbound one doesn’t (what, you never noticed?).

During the construction works, to save money, some of the spoil from the new tunnels was dumped into the now-abandoned tube tunnel closest to where the new Southbound tunnel was dug, and later both ends were sealed shut.

The new line opened in 1926, and the old loop was lost to history.

Getting back to 1940…

A week after the bomb hit the tunnel – on the 18th Sept — soundings were taken from a boat to locate the crater caused by the bomb impact, and two days later, divers were sent down to have a look at the damage.

The bomb impact in the riverbed was significant, with a crater that depth was about 15 feet below the bottom of the Thames, and the surrounding soil was piled up by about 5 feet around the crater.

The diving team examined the tunnel and found that 12-15 feet of the abandoned loop railway tunnel had been fractured. The diving team reported a “considerable quantity of ballast and mud” as laying in the tunnel to a depth of about 3-4 feet above the rail track level.

Some of the war documents I read when researching this article were still classified as Secret until the 1990s.

According to another letter, about 8 feet of the soffit (internal ceiling) of the tunnel was broken at about 60 feet from its intersection with the Northern Line southbound tunnel. The loop was flooded between Bulkheads Nos. 13 and 14.

It was recommended that the crater be filled in as soon as possible as further explosions in the area would transmit shock directly to the southbound tunnel of the Northern Line by “water hammer”. The effect would also be felt on bulkhead 13 that was adjacent to the fan shaft and if that had fractured, then the flooding could have spread to the rest of the Northern Line tunnels.

The under water work was carried out by Messrs. Shelbourne who plugged the ends of the ends of the tunnel with sandbags filled with concrete and filled the intervening space with ballast and “puddle clay” from a barge moored above the impact point.

This was completed on the 6th November.

Another company, Messrs. Mowless waterproofed the opening to a disused heading leading to the old Charing Cross station with aqalite and brickwork. This was completed on the 7th November.

An additional bulkhead was also erected about 16 feet west of bulkhead 13 and 5 feet west of the opening leading to the vent shaft to the District Line. Bulkhead 13 itself was a concrete wall, but had an inspection door to allow access into the tunnel for inspections, although a note on one map suggests the door was sealed permanently shut in 1939.

Bulkhead No. 14, which had been leaking water though a small ventilation pipe was waterproofed and fitted with a steel reinforcing plate.

The crater plugged, the tunnel was drained of river water and a pump installed to keep it dry.

Today

Just over 70 years later and the tunnel is still there, a ghostly relic of the early years of the London Underground and an often overlooked memory of the Blitz.

It seems to be completely sealed off and inaccessible, although presumably at some point people will have to go in and carry out structural inspections. Old tunnels made from rusting iron rings will collapse eventually, and you wouldn’t want to be in the area when that happened!

Other bombs:

On the night of the 8th Oct, men repairing leaks caused by that earlier main bomb impact reported hearing the sounds of nearby bombs.

Although eye witness reports confirmed that bombs had been dropped in the area, and the Seven Seas schooner reported being hit by a wave, subsequent surveys were unable to find any bomb crater in the river bed, although some bomb fins were later found in the Thames.

However, surveys of the tunnels found that the joints had been shuddered by an explosion nearby, which was estimated to have been caused by a 50kg bomb.

On 12th November 1940, a major bomb destroyed Slone Square station, and the report noted that the bomb came close to the King’s Scholars Pond sewer that runs over the tracks at the station. Although the sewer was not damaged, it is noted that floodgates had been installed at South Kensington and Charing Cross (Embankment?) station to mitigate the effects of a flood down into the lower tunnels if the sewer was breached.

Its interesting to see that the floodgates were not just protection against bombs in the river.

On Sunday 10th May 1941, an unrecorded number of bombs were dropped on the buildings between Belvedere road and the river (roughly where the South Bank Centre is now).

Both Northern Line tunnels showed signs of being shaken by the impacts, with minor damage reported as extending 93 feet southwards and 157 feet northwards from the impact points.

Aftermath

A report on the 28th Sept., to inspect unrelated damage to Chalk Farm also included some interesting comments about the use of anti-flood bulkheads on the Underground.

The damage at Chalk Farm was so significant that the Engineers were of the opinion that had it taken place under the river, water would have “poured into the Tube at many places”.

I couldn’t find the earlier document, but it seems that based on the greater depth of the tunnels at London Bridge, some sort of decision was taken the day before to relax the use of bulkheads during bombing raids at London Bridge/Bank to only when an impact had been detected in the near vicinity as opposed to closing them when the air raid sirens sounded.

As the report noted, by 28th Sept, some 520 bombs had been dropped on London Transport properties, and the inspecting officers said that the changes were a “grave and entirely unjustified risk”.

Postscript

Could the abandoned tunnel be restored into use?

In hindsight, had the original plans to extend the line to Embankment, with a couple of sidings been approved, those sidings would probably still be in use today as a convenient turn-around point.

But we ended up with a loop – which was later sealed off. It was however noted to have a wider diameter than most tube tunnels, being 12ft 9inches — although that was to accommodate a fairly tight turning angle — so modern trains might just squeeze though it.

It could be presumably be possible to drive a large caisson into the ground around the impact and “drill out” the section then replace it with new tunnel rings, and remove the internal protection walls. You would also need a new junction where the loop meets the replacement southbound tunnel as they run into each other.

Would that be useful to be able to turn around trains at that location? Frankly, probably not to the degree that would justify the fairly significant cost.

Thanks to the staff at the London Transport Museum, The National Archives and TfL’s Archive at Greenwich for helping with this article.