Multiplay Level: Highway Loop:

Go the Highway Loop Level and kill off about six to seven of the combatants. Drive to the raised, broken bridge (with the two health pick-ups and another pick-up in between), and find what looks like a power plant, directly off the road. There are about two to three giant steel balls there. Shoot the one closest to the bridge with a Gas Can (the projectiles don't work well for this), and it will break off and start rolling. Make sure to stay clear of its path, or you'll be squashed. Follow the ball and when it crashed through a wall, follow into the newly opened area and find the Black Cube inside. Health and weapon pick-ups are also inside. Black Cube opens Elevators Level in multiplayer.