Years ago, on a visit to the much missed Ordnance Collection at Christinia, Denmark, I was shown a large glass sphere, like an enlarged fishing float, which I was told was a British WW1 Gas mine, found in the Baltic. I was not sure if this was correct, but recently I came across a history of the US development of WW1 Naval Munitions, which mentioned an expert in Naval Gas Warfare had been seconded by the Admiralty! Short of overwhelming a rowing boat, I fail to see how a large enough concentration could be achieved. Perhaps the mine was filled with a liquid, which could evaporate over a large area of the sea? Anyone else know something about this subject?