What are anti-continents?

Answer 1

This could refer to the theoretical production of refractory melting residue in geology, said to be the antithesis of continental crust creation. Hence, the term "anti"continent.

Answer 2

Anti-continents refer to large areas of land that existed in the geological past but are now submerged under the ocean. These landmasses were part of the supercontinents that have been breaking apart and drifting over millions of years due to plate tectonics. Examples include Zealandia and the Kerguelen Plateau.