In school, we learned the continents: the Americas, Eurasia, Africa, Australia, and Antarctica—with island nations like New Zealand floating off by themselves.

So you may be surprised to hear that scientists have discovered a new continent—and New Zealand is part of it, after all. In fact, it’s one of the only parts of what’s now called Zealandia that’s above sea level.

Most of Zealandia, which is almost two-thirds as big as the continental United States, is more than a kilometer underwater.

That’s deep enough that you might be thinking, “That doesn’t sound like a continent.” Except the seafloor around it is 4 kilometers deep.

Antarctica, too, if you melted all the ice, would be just a few islands, with most of the western part of the continent submerged.

Which begs the question: what exactly makes a continent? The leading factors are its mineral content and density.

The seafloor is dense and heavy with iron, so it rides low in the mantle that supports all of Earth’s crust.

Continental crust is high in silica and aluminum. It’s less dense and much thicker than oceanic crust, so it floats higher in the mantle, like ice on a lake.

Most continents are large landmasses surrounded by a shallow ocean shelf.

But Zealandia is a small landmass surrounded by a large ocean shelf, having been pushed around, squeezed, then sunk by plate tectonic forces over millions of years.