Putting a steam engine inside a submarine is exactly as dangerous as it sounds. In World War I, the British Royal Navy wanted submarines that could keep up with battleships. Diesel engines were too slow, so they did the unthinkable: they installed massive steam boilers inside the hull. To work, these subs needed giant retractable chimneys on the deck. Before diving, the crew had to fold the smokestacks down and seal the exhaust ports. If a single valve failed, the ocean would pour straight into