The most formidable naval weapons in both world wars, German submarines devastated trans-Atlantic shipping while sinking more than 6,000 merchant vessels and warships and killing tens of thousands. These U-boats (an abbreviation of Unterseeboot, the German word for “undersea boat”) prowled the oceans in search of prey and could attack ships 20 times their size from both above and below the surface with their deck guns and torpedoes.

Inside the dimly lit, claustrophobic submarines, sailors couldn’t shower or even change their clothes during patrols that could last two months at sea. Fifty men shared two toilets—one of which doubled as a food locker at the start of patrols—that couldn’t function when 80 feet or more below the surface because of the outside water pressure, according to The U-Boats by Douglas Botting.

U-boat crews inhaled a foul cocktail of bilge water, sweat and diesel fumes. Mildew blossomed on their shoes, and charts even rotted from the oppressive heat and dampness. “I feel like Jonah inside some huge shellfish whose vulnerable parts are sheathed in armor,” wrote German war correspondent Lothar-Günther Buchheim on patrol in 1941.

Submarines were still primitive naval weapons when Germany became the last major naval power to build one in 1906. By the start of World War I in 1914, however, Germany had caught the competition. Its 20 combat-ready U-boats were more sophisticated that other countries’ submarines and could travel 5,000 miles without refueling, allowing them to operate along the entire British coast.

The U-boat fleet made its first strike on September 5, 1914, with an attack on a British light cruiser off the coast of Scotland that killed more than 250 sailors. Seventeen days later, U-9 sank three British battle cruisers with an hour, killing nearly 1,500. Despite these strikes, the Germans lost more U-boats than they sank during the first month of the war.

In February 1915, Germany announced the start of unrestricted submarine warfare in which all vessels, even merchant ships from neutral countries, would be sunk without warning in a war zone around Great Britain. The idea that submarines would attack merchant ships had been dismissed by many Britons, including First Lord of the Admiralty Winston Churchill who wrote, “I do not believe this would ever be done by a civilized power.”

U-boats attacked not only food and oil supplies bound for the British Isles, but passenger ships as well. On May 7, 1915, U-20 torpedoed the liner Lusitania off the coast of Ireland and killed nearly 1,200 passengers, including 128 Americans. Alarmed at the prospect of an American entry into the war, Germany eventually pledged to protect the safety of passengers before sinking unarmed ships.

The Allies struggled to counter the U-boat threat. The Royal Navy camouflaged warships with paint jobs to pose as merchant vessels and piled haystacks to obscure guns. Some British patrols even carried canvas sacks and hammers that they could use to cover and smash U-boat periscope lenses.

After announcing the resumption of unrestricted submarine warfare against Allied and neutral ships on January 31, 1917, U-boats sank more than 500 vessels by the end of April. The U-boats nearly succeeded in defeating Great Britain, but attacks on American merchant ships played an important role in the United States entering the war.

Germany’s development of the U-cruiser submarine allowed it to strike the Atlantic coast of its new enemy. The first German U-boat arrived in American waters in May 1918 and sank 13 ships—including six in a single day—in addition to laying mines in American ports and severing two telegraph cables on the seabed during its 12,000-mile patrol.

By grouping merchant ships in convoys and escorting them with warships, Allied countermeasures began to blunt the U-boats, although the German submarines succeeded in destroying more than 10 million tons of cargo by the time World War I ended.

Although the subsequent Treaty of Versailles required the surrender of all U-boats and prohibited their future possession by Germany, submarine construction resumed after Nazi leader Adolf Hitler repudiated the peace pact in 1935. World War I demonstrated that long-range submarines could be powerful weapons, and when war returned in 1939, so did the U-boats.

When World War II commenced, Germany had 57 submarines under the command of Commodore Karl Dönitz, who had served on U-boats in World War I. Dönitz believed the war would be decided in the Atlantic and that he could win it with 300 U-boats.

In May 1940, Hitler approved unrestricted submarine warfare on all shipping around Great Britain after initially rejecting the idea to avoid provoking the United States. Once in possession of ports in Norway and western France, Germany extended the range of its U-boats to disrupt merchant shipping. U-boats stalked their targets for days and attacked in groups that the British called “wolf packs.” From summer 1940 to spring 1941, each U-boat at sea sank an average of eight merchant ships a month in what Germany called the “Happy Time.”

Although the British implemented a convoy system at the start of the war, it was poorly protected for the first 18 months. Radar remained primitive. Aircraft were few in number, lacked sufficient range and couldn’t provide escort coverage at night. While the Allies lacked adequate intelligence on U-boat movements, Germany intercepted cables between American shipping insurance firms and European underwriters to learn about ship cargoes, sailing dates and destinations.

After the United States entered World War II, a wave of 16 U-boats attacked merchant ships along the American and Canadian shorelines as part of Operation Drumbeat. Taking advantage of weak and disorganized defenses, U-boats roamed as far as the Gulf of Mexico and cruised inshore shipping lanes during the first half of 1942. U-boats that lurked along North Carolina’s shipping lanes sank 78 merchant ships and killed 1,200 merchant marines.

Once American merchant ships began to sail in trans-Atlantic convoys with continuous sea and air escorts, attacks fell dramatically. Along with the breaking of U-boat ciphers, improvements in radar technology and the effectiveness of attacks by long-range bombers and escort carriers led to the sinking of 41 U-boats in May 1943, including eight in one day. Dönitz responded by ordering his submarines to retreat to more remote locations such as the Indian Ocean where targets would be unescorted.

U-boats returned to the British coast in 1944 after the development of snorkel ventilation tubes allowed them to operate longer and deeper underwater to reduce the chance of detection by radar and enemy aircraft. However, they suffered heavy losses and few successes. After Hitler’s suicide on April 30, 1945, Dönitz served as his successor and ordered German forces to cease operations and surrender. The 45 U-boats at sea surfaced and proceeded to ports designated by the Allies.

By some estimates, Germany lost three-quarters of the U-boats it built during World War II. Although they ravaged Allied shipping over the course of two world wars, the U-boats also became steel coffins on the ocean floor for approximately 30,000 of the 40,000 sailors who manned them.