From the back of the box.

"About the Boomers
Boomer submarines constantly prowl the ocean depths with their arsenal of nuclear ballistic missiles. These leviathans of the deep play a powerful role in your strategic forces.

Watch your foes sweat and yield to your command as they face the sudden threat of nuclear threat. Surprise, overwhelm and penetrate their laser star defense systems.

The critical edge is yours. The ocean depths conceal your boomers as they slip into position and sink enemy ships.

Now if your land based nukes are captured or destroyed, you'll always have a nuclear reserve lurking beneath the waves.

Playing Equipment
This expansion set contains new rules and 40 miniature boomer submarines made of durable plastic. There are five boomers color-matched for each superpower. Boomers have also been included for use with the Middle Powers expansion set. Each detailed miniature is 2 1/4 inches long (58 millimeters)."