Mariel, on the northwestern coast of Cuba, served as a major port of entry for Soviet goods and as a Cuban naval base. NPIC analysts monitored imagery of the port to gauge whether additional Soviet materials were brought in to add to the already growing threat. The Soviets attempted to send four “Foxtrot” diesel submarines to Cuba during the crisis. Intelligence gathering provided by NPIC and other agencies resulted in the U.S. Navy successfully forcing three subs to the surface before turning them around, with the fourth turning back in mid-journey, still submerged.