ground-state energy
The ground-state energy is the lowest energy a confined quantum system can have — and crucially, it is never zero. A trapped particle cannot be brought completely to rest the way a classical ball can settle at the bottom of a bowl. Even at absolute zero temperature, with all the energy it can lose drained away, a bound particle keeps a residual, unavoidable amount of energy. This stubborn minimum is also called zero-point energy.
The reason traces back to the uncertainty principle. Pinning a particle into a small region sharpens our knowledge of its position, which forces a corresponding spread in its momentum. A spread in momentum means the particle cannot have exactly zero motion; some unavoidable jiggle remains, and that jiggle carries kinetic energy. The tighter the confinement, the larger this irreducible energy — squeeze the box and the floor rises.
This is not a quirk but a load-bearing fact about nature. It is why helium stays liquid down to absolute zero instead of freezing, why electrons in atoms do not spiral into the nucleus, and why a vibrating molecule never stops vibrating entirely. The very stability of matter rests, in part, on the fact that confinement always exacts a minimum energy that cannot be removed.
The lowest level of a box sits above zero, and rises as the box L is made smaller.
Zero-point energy is real and measurable, but it is not a free fuel source. You cannot extract it to do useful work, because it is already the lowest possible energy of the system — there is nowhere lower for it to fall.