A high-inertia clarketech artifact of unknown origin discovered on Zeta Corianis IV. It is a small object, about 2 centimetres on a side but with the inertia of at least 8 trillion kilograms. One would expect it to generate a huge gravity field, very close to a black hole singularity, but there is no evidence of this.
The Null Box has three clarketech properties:
1) It is a solid not made of particles (at least, not as far as researchers can determine). 2) It has a fraction of the gravitic field one would expect from an object with such high inertia, and 3) It is apparently indestructible- it cannot be cut or chipped, or chemically interacted with etc.
The Null box is extremely hard to move not only because of its extraordinary inertia but also because it tends to sink into matter that's pushing against it.