[Maytime]

The lack of a secondary explosion leads me to believe that they just stuffed a bunch of explosives under ground and set it off, with the full knowledge that an underground explosion would be much harder to verify as nuclear.

I didn't see the seismograph data, but there should be a little tick before the reaction, representing the catalytic explosion.

Also, underground nuclear testing leaves subsidense craters, which one could measure (at a given depth and ground material) and determine the yield of the device. Notice in the picture (of the Nevada nuclear test site) the many circles, which were tested from 100m-500m deep IIRC.