Often abbreviated to MK48, this torpedo is worthy of the name 'heavyweight.' Its bulk means it's only effective at the comparatively short range of five miles, but when it slams in its target at 30 miles per hour, it detonates a 650lb warhead.
You'll find MK48s aboard every American submarine currently in service, and they can be directed toward other submarines, or enemy ships on the surface. Homing technology means that they're highly unlikely to miss once they've been launched.