[Cyber Liberty]

My knowledge of geography is lacking. I didn’t know there were islands large enough to have enough volume underground to contain blasts.

[Tijeras_Slim]

They would drill down about a half mile into the center of an atoll, through a limestone layer into basalt. That would give them a substantial amount of overburden for containment. Here's a pic of one of their tests.

Even after the ban on atmospheric testing, everyone interpreted what a "release of radiation" meant differently. A number of containment failures have occurred.