When you begin Act 5 at the bow of the Outer Haven submarine, there will be two large vents protruding from the floor nearby to the right. Avoid the frog soldiers surveying the area and stand atop one of the vents. You might see items below if you peek through the grid. To the left of your starting point will be a hatch in the floor. Crouch over it to get the option to perform a special action. Press Triangle to have Snake open the hatch, then drop in. Note that the hatch and the area beneath deck are difficult to see if you do not have your night vision switched on. Crawl through the vent into the space under the first vent. You will find the "Metal Gear Solid Main Theme" iPod song. Continue crawling into the next space and you will find an RPG-7. On the far side of the crawl space, there will be another hatch that Snake can use to return to the sub's deck. Snake will emerge in perfect cover behind some containers, but the Frogs might hear the hatch closing and inspect your area.