The US Air Force's mysterious X-37B completed its most recent mission over the weekend, landing at California's Vandenberg Air Force Base over the weekend after spending the last 674 days in space. What it was doing up there, though, remains a mystery.

The X-37 originally launched from Cape Canaveral, FL in December of 2012. Along with the other Orbital Test Vehicle (as it's officially known) in the Air Force's fleet, the mysterious shuttle-like crafts have spent over 1,300 days in orbit.

"Technologies being tested in the program include advanced guidance, navigation and control, thermal protection systems, avionics, high temperature structures and seals, conformal reusable insulation, lightweight electromechanical flight systems, and autonomous orbital flight, re-entry and landing," an Air Force statement said, according to CNN.

While we can speculate about the X-37's mission until we're blue in the face, few things actually make up for seeing the plane in action, so take a look at video of the elusive craft making its landing last weekend at Vandenberg.

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