In this 2007 file photo provided by the U.S. Navy, the research submarine NR-1 is moored at U.S. Naval submarine base, in New London, Conn. The nuclear-powered NR-1, launched in Groton in 1969, was one of the most secretive vessels in the U.S. undersea force.

GROTON, Conn. - It could dive deeper than any other submarine, and when it reached the ocean floor, the one-of-a-kind Navy vessel could roll on wheels with lights illuminating the depths outside its windows.

The nuclear-powered NR-1 was launched in Groton in 1969 and was one of the most secretive vessels in the U.S. undersea force.

It was known primarily as a research vessel, but it also carried out a range of Cold War military missions that remain secret today. Veterans who served aboard the 140-foot-long sub say it was one of the most fascinating assignments of their careers, but not even their wives know all the details.

The NR-1 was taken out of service in 2008, and the Navy has collected pieces of it for an exhibit at a submarine museum in Groton.