The continents, "floating" on the earth's denser interior, have sunk as much as two miles below their "proper" height, according to a report in the February issue of Geophysical Research Letters. ... It has long been assumed that the continents float on the underlying rock, just as an iceberg floats in water.

Is it possible for a continent to sink?

For a continent to sink you either need to add mass to the continent (pushing down on the mantle causing a diverging mantle flow underneath) or you need to push mantle from underneath the continent away from internal forces.

Are there any continents underwater?

Scientists confirmed the existence of an eighth continent, called Zealandia, under New Zealand and the surrounding ocean in 2017. Because 94% of Zealandia's 2 million square miles are underwater, mapping the continent is challenging.