Terra Nullius
This is Latin for “no mans land.” It’s a concept in international relations that refers to physical land that no nation claims. It can be claimed by occupation (though that might violate other laws).
The only major area of unclaimed land on Earth is Antarctica.
There are, at any given time, several smaller areas of terra nullius, generally caused due to border disputes (where claiming land would require acknowledging an undesirable border).
Why I Looked It Up
It came up in The Deepest Map when discussing how China was creating new islands in the South China Sea by dumping sand on the seafloor until it broke through the surface, enabling them to claim that “land” and the surrounding ocean.
Accretion is another time-honored tactic for claimining land by stealing it from the seafloor – the new terra nullius.