In a Swamp

Place In a Swamp in your command zone when the game starts with a Swamp counter on it.

When a land enters you may move the Swamp counter onto that land. When the land leaves play with the Swamp counter on it or the Swamp counter is removed return the counter to In a
Swamp.

Lands with Swamp counters on them count as Swamps in addition to any other types.