Glacial Crasher

Trample (This creature can deal excess combat damage to the player or planeswalker it's attacking.)
This creature can't attack unless there is a Mountain on the battlefield.

The List

Magic 2015

Trample (If this creature would assign enough damage to its blockers to destroy them, you may have it assign the rest of its damage to defending player or planeswalker.)
Glacial Crasher can't attack unless there is a Mountain on the battlefield.

If the only Mountains on the battlefield leave the battlefield after Glacial Crasher attacks, it stays attacking.

It doesn't matter who controls the Mountain.