I attended the navy day celebrations and the marine commandos demonstrated a bomb blast by setting a time bomb in a small installation at the middle of the sea. The commandos dived from their boat and set the explosives from underwater. After the explosion, what I saw first was a cloud of black gas which soon took the shape of a mushroom. Then after some time you could only see a dark ring of smoke above the location which remains for some time. How can you explain the two phenomena? Why does it take the shape of mushroom? Why not a sphere or cube? Why does it change into a large ring of gas which floats?