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

Why is sundering titan banned?

Sundering Titan is a mass land destruction spell that's very easy to keep looping, which is enough justification to have it banned. This card is extremely unfun to play against, especially as it'll punish casual budget mana bases just as much as the player with the mana base worth more than a car.

Does Sundering Titan destroy your own lands?

All the lands you pick get destroyed. If you can't find a land of a certain type (Say there are no swamps or plains on the field) then you can't choose a swamp or a plains, but you still find one of every basic type that is on the field, and destroy those targets.