Impressive photo of the giant tubes that house four of the 24 Trident II D5 Submarine Launched Ballistic Missiles (SLBM) carried by Ohio-class nuclear submarines. 14 of these submarines travel for months underwater, carrying fifty per cent of all the active thermonuclear warheads in the US arsenal.
The Ohio-class submarines will be replaced by a next-generation strategic nuclear submarine around 2031. Not many details are known about the Ohio replacement except that there will be 12 of them, they will be in service until 2080, have stealth capabilities, have 16 of these tubes, and they will be less expensive to maintain than Ohio-class subs:
The Navy recognises that replacing the Ohio-class submarine will have a large impact on the Department of the Navy shipbuilding budget, as SSBN procurement is a significant investment made once every ~40 years. However, the Navy is actively working to reduce costs and has already reduced approximately $US1.1 billion in construction per ship and ~$US3 billion in design from its fiscal year 2011 plan (calendar year 2010). The design incorporates a nuclear reactor that will not require refueling, enabling the planned force of 12 Ohio-replacement SSBNs to provide the same at-sea presence as the current force of 14 SSBNs, and saving taxpayers $US20 billion (calendar year 2010) over the life of the class.
Here's a first block Ohio-class submarine being refurbished to carry Trident II D5 missiles, so you can see the incredible size of the beast that has to hold the tubes above:
And here's the USS Michigan: