Transcript for ABC News’ David Muir gets exclusive access on USS Florida
We turn tonight to an ABC news exclusive. This evening, we take you onboard a nuclear-powered U.S. Navy submarine, a guided missile submarine nowen 0 a classified mission tonight. We were there in the eastern mediterranean, where they're not the only ones in the sea. Tonight, the tomahawks, the nuclear reactor, and the warning about the Russians, right there, too. 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 out there. Reporter: A nuclear-powered U.S. Navy guided missile submarine. 160 crew onboard. But they call it the silent service for a reason. That's right. 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 Houston describes we're standing 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. Reporter: And how many tomahawks are we sort of 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: And 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. Every inch of the submarine is used. They give me a harness -- Your head's going to go right in between there. Reporter: -- As we prepare to climb to the top of the sub to the bridge. We wait for word. Control bridge, send ABC to the bridge. Reporter: Up the ladder through several hatches. They tell you, when it comes to your hands and feet, make sure three out of four are touching at all times. We climb to the top, where they are on patrol as the 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 -- Is seven missiles. Reporter: Seven tomahawk missiles. Right. Reporter: And we take note that in between, the curtains drawn, where the sailors sleep. So the sailors are actually sleeping in between the tomahawk missiles. Right. Reporter: And inside the submarine the control room, they order the submarine is about to descend. Submerge the ship. Dive, aye. Dive, dive. Reporter: A camera shows the submarine disappearing under the water's surface. Eventually 400, 500, 600 feet beneath the surface of the sea. They have trained to move the submarine as carefully and as quickly as possible. 13 degree up angle. Reporter: Soon, we are all leaning with no effort. It is precision work in these waters of the eastern mediterranean. They have to be ready. There are others here, too. We've put this submarine right in this eastern portion of the mediterranean to counterbalance the Russia buildup in Syria. Reporter: Do you have company here in the mediterranean? We do have plenty of company. The Russians are very active and -- we're active with them. 2-7-0 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. Yes. Reporter: -- Into Syria, and now the Russians are here in the mediterranean? They absolutely are, and we're watching them very, very closely. Reporter: You are. Yeah. There's really not a day where we're not watching them every single day. Reporter: Are they watching us? I think they'd like to watch Reporter: In fact, the Russians recently showing their own underwater muscle in the barents sea. And just days ago, testing they're new sea-based ballistic missile. But 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. Initial search complete. No close contacts. Reporter: 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. Coming through. Reporter: And soon, we are standing in front of the hatch they are sealed off. We're basically traveling on the submarine with a nuclear reactor. We can operate more than 90 to 120 days submerged, and the reason is because that reactor gives us all the power we need. Reporter: And we ask who is behind the hatch. So, the team back there is about 11 watch-standers, highly trained nuclear operators. Reporter: And this submarine has only been refueled once? Only once. Reporter: And that nuclear power also produces oxygen on board while under the sea. When you're 500, 600 feet below the surface, you have to use the resource you have, which is water. Right. We're breaking down deminerazed water into oxygen and hydrogen. Reporter: And there is something else about the "Uss Florida." It is always ready for U.S. Special forces, for Navy S.E.A.L.S. Their weapons already onboard. 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. And 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. And onboard, there is one more powerful weapon. The torpedo. As a captain, you always want to be ready. Reporter: So this one here? Right. Reporter: They have eight of them. One already loaded. Captain Burton is the commanding officer of the you just look at the region, and you've got ISIS in northern Africa, you've got what's going on on the turkey/syria border right now. The fact that you're here, in the mediterranean, does that give you a set of silent eyes for the U.S.? Absolutely. It gives them eyes where no one knows that they're being looked at. Reporter: And tonight, the "Uss Florida" now on that classified mission. Nobody knows where it's at at the mediterranean at any one time. Reporter: Including now. Including tonight. Tonight, all we can report here is that classified mission does continue, but the rear admiral and the captain made it clear to me, given the region that they're in, the eastern mediterranean, there is plenty for them to track there, plenty of intelligence to send back to Washington.
This transcript has been automatically generated and may not be 100% accurate.