If you stuffed that box with neutronium then:

1. Funny event: it's so dense the Earth itself is basically a thin gas in comparison and it immediately falls through the floor, the ground, and the mantle to oscillate around in the core.

2. Funny other event: It's so massive it dominates gravity nearby and everything within a couple of meters gets turned into Cool Physics from aggregating onto an incompressible box really fast and hard. Maybe the nearby atmosphere ignites from being compressed into plasma against the box.

3. Real physics step in and the neutronium immediately decompresses and the mass equivalent of an inland ocean in neutrons and angry high-energy high-mass decay products sterilizes everything through to the horizon with a gamma ray burst, also triggering massive seismic events from the blast as well as killing everything on Earth since the atmosphere is now radioactive and a lot thinner