We had one of those near where I used to live in England, about four or so years ago.
The cause of that one was petrol overflowing a storage tank after automatic monitoring systems broke down. Petrol started flowing out of vents at the top of the tank, turning into a flammable vapour cloud as it cascaded down the side of the tank.
The gaseous cloud covered nearly 8 hectares of ground by the time it was ignited, possibly by a spark in a fire pumphouse or an electrical generator building close to the site.
I don't think anyone died, which was a miracle and there were no long-term public health problems.
A lot of my friends were awoken by the blast which blew their loft hatches down!
It measured 2.4 on the Richter scale.