To survive Nick must have oxygen. The oxygen level can be recharged by collecting oxygen items or surfacing the main sub. Collecting a shield item increases the shield level, but it decreases the torpedo weapon by one level. The pulse cannon can be obtained by collecting four parts (one from each following level). The pulse cannon is powerful, but it uses up oxygen. In most levels, the green key must be collected to access the route to the exit. Treasures can be found either floating around or contained in barrels and chests. When faced by a boss in each episode, a weak point must be hit repeatedly to destroy it.