This skull is located on the mission "Halo", right after you first get the warthog. Instead of going downhill to enter the Forerunner cave, go right until you are above the cave. The Skull will be on one of the cliff ledges.
When Boom is activated, all explosions have double the explosion radius. As well as grenades and explosive weapons, this also affects Grunt Funeral explosions and ambient explosions such as those on The Maw.