Thanks Lu,
I'm not entirely sure about the relative sizes of the explosions, you might well be correct. The WW1 trench explosion and its aftermath sounds very similar to the Fauld disaster, where the water from a 6 million gallon reservoir went into the crater and many unexploded bombs etc are thought to litter the area to this day. I fly over this area regularly and the local fields seem to have more than their fair share of ponds. I think their origin was possibly disturbed ordnance from the original explosion falling into the surrounding fields, certainly the RAF ground to air photo taken the folowing day shows many small craters around the main one.
I have been told of another similar "Danger" site on the north west coast of England where there used to be nitro-glycerine production plant (I think it's Cumbria, but the chap who told me wouldn't divulge exactly where, he said he had worked there for a while and his contract forbade it).
Apparently, due to a plant leak which continued undiscovered for a long time, many tons of this stuff is now festering quietly away under the sand and again, no-one knows if one day there will be a large explosion. The area has been cordoned off for many years, to protect public safety.