What is an antonym for archipelago?

There isn't a single perfect antonym for "archipelago," as the word itself describes a specific geographic feature. However, depending on the context, some suitable alternatives could be:

Focusing on the "group of islands" aspect:

* Continent: A large landmass.

* Mainland: The principal landmass of a country or region.

* Single island: Emphasizing the contrast with a group.

* Unbroken coastline: Describing a continuous stretch of land bordering water.

Focusing on the "spread out" aspect:

* Concentrated: Describing something as being clustered in one place.

* Compact: Describing something as being densely packed.

Focusing on the "oceanic" aspect:

* Inland: Describing something located away from the sea.

* Terrestrial: Relating to the Earth's land surface.

The best antonym for "archipelago" will depend on the specific context and the emphasis you wish to convey.