Transcript for A rare look inside nuclear powered submarine USS Florida
Reporter: We approach the "Uss Florida" in the eastern mediterranean as it prepares for a highly classified mission. So this is 18,000 tons we're looking at. 18,000 tons of American steel. Reporter: A nuclear-powered U.S. Navy guided missile submarine. 160 crew on board. They call it the silent service for a reason. That's right. All those missions are some of the most highly classified that we do. Reporter: They deploy underwater for up to 120 days, several months at a time. We are given rare access as we board the submarine. Thank you. Reporter: Rear admiral William Huston describes where we're standing, just above the tomahawk missiles. This can lift up at any time? This could lift up at any time on order if we wanted to launch the tomahawk missiles. And how many tomahawks are we standing on right here? Right now seven. Reporter: They have more than 100 tomahawks at the ready. And he points out something else. We won't go any further. But you're literally standing 10 to 20 feet from an operational nuclear reactor right now. Reporter: They are about to take us down into the submarine, where we will spend the next 24 hours traveling with them. A maze of narrow hallways and hatches. Hi. How are you? Good to see you. Reporter: Every inch of the submarine is used. They give me a harness. Your head's going to go right through there. Reporter: As we prepare to climb through the top of the subto the bridge. We wait for word. Bridge, send ABC to the bridge. Reporter: Up the ladder through several hatches. They tell you when it comes to your hands and your feet make sure three out of four are touching at all times. We climb several floors to meet the captain and members of his crew waiting atop the "Uss we make it to the bridge, where we find they are on patrol as submarine leaves port. Preparing to descend into the sea. Back down inside the submarine, captain Seth Burton takes us past the missile tubes, holding the tomahawks. So inside this tube right here -- Seven missiles. Seven tomahawk missiles. Right. Reporter: And we take note in between the curtains drawn where the sailors sleep. The sailors are actually sleeping between the tomahawk missiles. Right. Reporter: Inside the control room they're about to send the order to descend. Submerge the ship. Dive. Dive. Reporter: A camera shows the submarine disappearing below the surface. Eventually submerging 400, 500, eventually 600 feet below the surface of the sea. They train to move the submarine as carefully and as quickly as possible. Soon we are all leaning with no effort. It is precision work in these waters of the eastern mediterranean. Not long after, we are leaning in the other direction. This is a 20 degree lean, right? Yes. Reporter: As they pull back. They have to be ready to make these moves. There are others here too. You put this submarine right in the eastern portion of the med franian to counterbalance the Russian build-up in Syria. Do you have company here? We have plenty of company. The Russians are very active and we're active with them. 2-0 to the left. Reporter: The U.S. Aware the Russians are trying to send a message. The Russians have demonstrated their willingness to use missiles from submarines. They did it from the black sea into Syria and now the Russians are here in the mediterranean. They absolutely are. And we're watching them very, very closely. You are. Yeah. There's really not a day we're not watching them every single day. Are they watching us? I think they'd like to watch Reporter: In fact the Russians recently showing their own underwater muscle. In the bering sea. And just days ago testing their new sea-based ballistic missile. It's not lost on anyone what we've seen from the Russians in just the last week and a half. Absolutely. And that's one of the reasons why we're here. Reporter: At night we watch as they use a periscope with an infrared camera above the water. No contacts. So we're alone. We are alone. Nobody's there. Which is good. Reporter: We crawl through another hatch and snake our way to the nuclear reactor. Coing through. Reporter: And soon we're standing in front of the hatch they have sealed off. We're basically traveling on a submarine way nuclear reactor. Absolutely. We can operate more than 90 to 100 days submerged and the reason why is that reactor gives us all the power we need. Reporter: And we ask who is behind the hatch. The team back there is about 11 watch standers. Highly trained nuclear And the submarine has only been refueled once? Only once. Reporter: And that nuclear power also produces oxygen on board while under the sea. We are taken to the room where they monitor it all. When you're 500 or 600 feet below the surface you've got to use the resource you have, which is water. Right. We're bringing down demineralized water into oxygen and hydrogen. We make approximately twice as much hydrogen as we do oxygen. We then send that hydrogen overboard. When it hits the electricity it breaks it into hydrogen and oxygen. Yes, sir. Reporter: There is something else about "Uss Florida." It is always ready for special forces, Navy S.E.A.L.S. Their weapons already on board. We are about to climb to see the small compartment attached to the top of the submarine where Navy S.E.A.L.S, U.S. Special forces would deploy right into the water. Keep in mind the submarine is still hundreds of feet beneath the sea. This is the dry deck shelter on top of the submarine. In fact, we're still about 200 feet beneath the surface of the mediterranean here in this room. This is where the Navy S.E.A.L.S would deploy in a Navy S.E.A.L. Delivery vehicle of some sort. In fact, the only thing separating me from the intense pressure of the water is this black hatch. And in fact, if you listen you can actually hear the water. Reporter: And on board there is one more powerful weapon. The torpedo. As a captain you always want to be ready. So this one here? That's right. Reporter: They have eight of them on board this submarine. One of them already marked war shot loaded. And it turns out that every one of these torpedoes is tested multiple times before it's put into place on a guided missile submarine. These torpedoes were shot five times successfully as an exercise torpedo. The engine compartment, the after bombing and the quord gnat nose group, the guidance control group of the weapon before it can -- these weapons actually have run time. Reporter: Petty officer izamar drake conducts a test run of the tube itself. Shooting 2-4! So the force that was getting released in there was the 2,000 pounds of air going in through the firing valve into the turbine ejection, which is pretty much like a jet engine spinning it up, which just ejects it out like a giant water We have torpedoes that will basically eliminate any submarine or surface ship if needed. If needed. If needed. Reporter: Captain Burton is the commanding officer of the "Uss Florida." You just look at the region and you've got nicis ISIS in northern Africa, what's going on on the turkey-syria border right now. The fact you're here on the mediterranean, does that give you a set of silent eyes for the U.S.? It gives us eyes where no one is aware they're being looked at. Reporter: Captain Burton is ware of the Russian tooz. And the Russians have found a way to signal to you we're here in the mediterranean too. Oh, definitely. They're here. And do you let them know that you know they're here? That's why I'm here. Reporter: And on this submarine as we walk down those narrow hallways, just enough room to get by, these sailors often do not know the classified mission on the horizon. They do know they're here in the eastern mediterranean and that they're ready. All right. Good evening, everyone. Good evening, sir! You all are about to go on some missions that I can't tell you about here on the mess decks. You're in a very dangerous part of the world right now. Reporter: And tonight the "Uss Florida now on that classified mission. Nobody knows where it's at in the mediterranean at any one time. Including now. Including now. Reporter: I'm David Muir for "Nightline" in the eastern mediterranean. The rear admiral telling David the submarine has three times the firepower of a typical Up next, "The little
This transcript has been automatically generated and may not be 100% accurate.