Sundering Titan

When Sundering Titan enters or leaves the battlefield, choose a land of each basic land type, then destroy those lands.

"Even if we defeat it, what remains to be saved?"
—Glissa Sunseeker

The Brothers' War Retro Artifacts

"Give the schematic to the younger brother. Tell him that it will surely keep Argoth's resources out of Urza's hands."
—Gix, Yawgmoth praetor

Double Masters

When Sundering Titan enters the battlefield or leaves the battlefield, choose a land of each basic land type, then destroy those lands.

Masters 25

If a land has more than one basic land type, it can be chosen more than once.

If one of the basic land types isn't present, it isn't chosen. If the only land of a certain type is one you control, you must choose it.

Players can't take actions in between the time you choose the lands and the time you destroy them. Notably, they can't activate mana abilities of those lands.

Sundering Titan's ability isn't targeted. When it resolves, Sundering Titan's controller must choose one land for each basic land type (Plains, Island, Swamp, Mountain, and Forest), and then they are destroyed simultaneously.