The Boomers expansion set allows players to add ballistic missile subs to their navies. Players must pay $2 Billion dollars and lose one grain, one oil, and four minerals units from their supply center to build each sub. Each sub carries two missiles and can be used for conventional or strategic attacks. The rules contain specific info for offensive and defensive tactics. The expansion set contains 40 color-coded miniature subs, five per-superpower.Expands: Supremacy - BoardGameGeek