Zealandia

 Zealandia was originally part of the ancient supercontinent of Gondwana, which was formed about 550 million years ago and essentially lumped together all the land in the southern hemisphere. 

It occupied a corner on the eastern side, where it bordered several others, including half of West Antarctica and all of eastern Australia

Another mystery is exactly when Zealandia ended up underwater – and whether it has ever, in fact, consisted of dry land. The parts that are currently above sea level are ridges that formed as the Pacific and Australian tectonic plates crumpled together. Tulloch says opinion is split as to whether it was always submerged apart from a few small islands, or once entirely dry land.

 

https://www.bbc.com/future/article/20210205-the-last-secrets-of-the-worlds-lost-continent