system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Underground Nuclear Tests

Where does all the energy from the testing go ? Is the underground test an actual full scale explosion ? I read that North Korea most likely did their Hydrogen Bomb test underground which caused a 5.1 reading on the Richter Scale.
gpt
Underground Nuclear Testing is basically burying the bomb very deep underground to help ensure that none of the radioactive fallout goes into the atmosphere. The main idea is environmental protection. There are no special big gigantic underground laboratory structures that the bomb goes off in... it's more or less just a hole in the ground (although generally there are still sensors everywhere to test things).

When the bomb goes off, all of the energy is dissipated into the ground around it. The dirt/rock directly next to the bomb will be vaporized creating a cavity, and a much greater portion of the rock/dirt will be cracked, crushed, and/or undergo irreversible changes from stress. Often the top of the ground will be partially blown upwards, and the cavity can collapse forming a crater.

Large underground explosions, nuclear or otherwise, can be read by seismographs as they are more or less causing earthquakes (although by a completely different method than natural earthquakes) so the 5.1 estimate of North Korea's bomb means it released a similar amount of energy into the surrounding earth that a 5.1 earthquake would have. It's also important to note that data about what happens in North Korea is sketchy, so there is no definite evidence that it was actually a nuclear bomb or something else that North Korea took credit for (like, say, an actual earthquake). However, many other nuclear tests have been detected using seismographs and in some cases they were published as earthquakes by public media first before anyone learned a bomb had gone off.