What would happen if 1 kg TNT exploded 1 km underwater?

The usual charge was on the order of 100 kg (much larger than 1 kg) but at depths of 250–300 feet (much less than l km (about 3000 feet). Thus you have a much smaller explosive effort working against much greater water compression. read more

The mass of TNT would be several gigatons, somewhere between 3 and 50 depending on how expansive you want to be in defining what constitutes 'Mount Everest'. This is on a par with the Tambora eruption (33 gigatons). read more

Umbrella test 50 meters underwater: "An underwater crater 3000 feet across and 20 feet deep was produced." I crudely estimate that a 10 Mt device .5 km underwater would displace/eject at least some 5-10 cubic kilometers of water, creating a huge "depression" or "cavity" in the ocean surface while ejected water experiences a sudden trip to stratosphere. read more