When this cavernous, triple-domed building was completed in 1917, its reinforced-concrete shell-frame construction was unique in the world. Resembling a classic Bond-villain lair, the vast space was completely restored and opened to the public in 2012 as a museum celebrating Estonia's rich seafaring heritage. Highlights include exploring the cramped corridors of a 1930s naval submarine, an elegant collection of ice-yachts, hanging from the ceiling as though in flight, and the many interactive exhibits to try your hand at.
Don't overlook the antique ships moored outside, including the handsomely-maintained Suur Tõll, the world's largest surviving steam-powered ice-breaker, which you can explore from stem to stern.