WHAT DOES UNDERWATER NUCLEAR TEST 'UMBRELLA' MEAN?

With test moratoriums over the horizon, American weapons labs rushed out numerous new designs. A hard deadline for testing was established at midnight (0000 hrs), 31 Oct 1958, as negotiations were being established to start that day, as well as the schedule displays it, with 29 tests executed in Oct, 4 of them on the final day. 1 other test was can