On the 27th November 1944 3,500 tons of high explosives and an estimated 500 million rounds of rifle ammunition exploded underground at the RAF's munitions dump at Fauld in Staffordshire. 
The likely cause of the explosion was a spark from the brass chisel being used by an airman on a live bomb - a tool that was not permitted, but probably made the job easier. This spark set off a chain reaction amongst the live munitions. A nearby farm and lime works were completely destroyed and virtually every house in Hanbury village was seriously damaged. A 6 million gallon reservoir was breached, the flood waters causing severe damage. Seismographs across Europe registered the blast. Had the blast occurred above ground, the effect would have been similar to that of the nuclear explosions at Hiroshima and Nagasaki.
The official inquiry found that there were several systemic failings that had contributed to the accident: management of the site was poor, allowing dangerous work practices to exist. Suitable manpower had been in short supply, so Italian prisoners of war (200 of them) were being used to carry out menial tasks. Rescuers were met with confusion; no-one knew who was trapped underground because the required register wasn't maintained.
Approximately seventy eight people were killed, mostly civilians in the nearby plaster works and local people.

The crater is 90 feet deep and covers 12 acres.
Despite the scale of the disaster, few people have heard of it. You can read more about the explosion at this local history website, and more images are available here.

Today marks the anniversary of a serious accident on the Great Western Railway at Norton Fitzwarren in 1940. 27 people were killed, and a further 75 were injured when their express sleeper train from London (carrying over 900 people) passed two signals at danger, derailed and overturned. The accident occurred at about 3.45 a.m. on a very dark, wet and windy night. 
The train had been routed from the main line to a relief line at Taunton, to allow another train (carrying newspapers) to pass. However, the driver was under the impression that he was still on the main line and continued to accelerate until he realised his error. By that time it was too late to bring the train to a halt. The train went through a set of catch points at about 45 mph (there to protect the main line). The locomotive tipped onto its side and the first six coaches telescoped into each other, blocking all four tracks. Luckily, the newspaper train had just passed the express - had the two trains collided the casualties would have been far greater. 
The causes of the accident still feature in rail accidents today. The driver, with over 40 years' experience, was probably operating on 'auto-pilot' - his experience worked against him as his actions became subconscious; including cancelling two warnings from the automatic signalling system. He had also lost his 'situational awareness', being unaware of which line he was running on; he had never before been diverted onto this line and the signals that applied to him were on the opposite side of the track from normal practice. Fatigue and other psychological factors were also likely to be present; the train was working during wartime blackout conditions, during the night and his home in London had recently been damaged by bombing.
You can read the accident report on the Railways Archive website, and the story is told in detail in this contemporary newspaper report.