Seven miles west of Mystic Seaport, GROTON is a suitably unpleasant name for the hometown of the hideous US Naval Submarine Base , headquarters for the North Atlantic fleet. The USS Nautilus , the country's first nuclear-powered submarine, was built in Groton. In 1958, four years after it was launched, it became the first vessel to sail under the polar icecap. It's now moored on the Thames, and self-guided tours allow access to its terrifyingly claustrophobic corridors, one-person-wide in many places. The sub looks pretty much as it did in the 1950s, complete with pin-ups of Marilyn Monroe. The Submarine Force Museum next door has exhibits on the history of submersibles from the minuscule American Turtle , built in 1775, to the frighteningly powerful Trident (mid-May through Oct Mon & Wed-Sun 9am-5pm, Tues 1-5pm; Nov to mid-May Mon & Wed-Sun 9am-4pm; closed first full week of May and last full week of October; free; tel 860/694-3174 or 1-800/343-0079, ).