* Photo: 3. Modernmechanix; 4: James S. Ketchum, MD; 6: USAMHI * 1. Ekranoplan

Dubbed the Caspian Sea Monster, the USSR's top-secret ekranoplan skimmed on a pocket of air just above the water's surface and well below radar. Only a handful were built, and the behemoth never made it out of trials.

2. Marine Corps Space Plane

A few years ago, the Marine Corps proposed Sustain — the Small Unit Space Transport and Insertion project — which would put boots on the ground anywhere in the world within two hours. It made a great PowerPoint presentation.

3. Atomic Airplane

In the 1950s, a rumor spread that the Soviets were working on a nuclear-powered aircraft. Alarmed, the US rushed to finish its own. The hitch: The crew would be exposed to dangerous levels of radiation. One solution offered was to use older pilots.

4. Psychedelics

For years, the US military experimented on its own soldiers with LSD, pot, and 3-quinuclidinyl benzilate, hoping to learn how to give foreign armies a bad trip.

5. Sun Gun

The Nazis developed plans to build a space-based reflector that would redirect sunlight into a devastating heat beam. Allied engineers doubted the Germans could do it. They were right.

6. Davy Crockett Nuclear Bazooka

A nuclear recoilless rifle of questionable accuracy — what could go wrong? It was actually deployed in Europe during the Cold War.

7. Excalibur

This space weapon would have used a thermonuclear weapon to power a massive X-ray laser. Eventually the notion of setting off hydrogen bombs in orbit started to seem like a bad idea.

8. Bat Bomb

Aiming to end World War II, the Pentagon sponsored a project to release time-bomb-laden bats over Japan. The mammals would nestle in the nooks of buildings, where their incendiaries would eventually ignite. Little Boy and Fat Man beat them to the punch.

9. Crusader

The Crusader could have shot 10 artillery rounds a minute at ranges in excess of 20 miles. It would have been awesome — for fighting the Soviet Union, which collapsed a decade before the super howitzer was finished.

10. Puckle Defence Gun

In the 1700s, Englishman James Puckle invented what many credit as the first machine gun. One selling point: It fired round bullets for shooting Christians and square ones for shooting Muslims, whom he felt deserved a more painful death.

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