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Baxter the robot moves on one big leg. BigDog has four. Curiosity roves Mars on six.

However, there’s a breed of robots out there that throws out legs altogether. Spherical robots are currently in use for research, fun and even surveillance purposes. Without legs they are inherently stable and capable of traveling great distances in short periods of time. Here are five of the craziest rounded robots rolling their way across Earth.

Tumbleweed

For more than a decade, NASA scientists have been working on a lightweight, inflatable ball to send to Mars. Wind would propel it across the surface at great speeds, allowing it to cover much more ground than today’s rovers, which usually stick to a range of just a few miles.

Early prototypes failed completely. A foot in diameter, they soon become stuck against rocks similar in size to those that litter Mars. Then, one day in the Mojave Desert, a wheel broke loose from an experimental NASA rover. The wheel, which was a giant inflatable ball, soared away. A 20-mile-per hour wind was enough to drive it over rocks and cliffs.

The revelation that a larger ball could overcome obstacles led to the development of Tumbleweed, a six-foot inflatable rover that has collected environmental data in Antarctica and Greenland. During one seven-day trip in Antarctica, it traveled 80 miles at an average of 3.7 miles per hour.

Phobos hybrid

Stanford University took the Tumbleweed concept one step further with its idea for a hybrid spacecraft/rover designed to roam Mars’ moon Phobos. Instead of an inflatable ball, its electronics would be sealed in a container covered in spikes. The spikes jut out to create a spherical shape and make contact with the ground as the robot rolls or lands.

At 1.3 feet across, the rover would not be suited to windblown tumbling like Tumbleweed. But it would be capable of long-distance jumps, and even high-altitude flight, which could take it across a small body like Phobos in a relatively short period of time. Rolling on the ground would be used for short-range travel only.

ROSPHERE

Small, plastic and purple, the ROSPHERE looks like it might contain a hamster or a rabbit. But instead of a furry pet, the robot holds a pendulum that shifts its center of mass to move it straight or in a curve.

Its developers at the Universidad Politecnica de Madrid built it to be rugged and particularly adapted to tougher terrains. For example, ROSPHERE can roll through rows of crops and gather environmental information about soil and plants. The data it generates could be of particular use to farmers and scientists interested in precision farming.

MorpHex

MorpHex is made of 12 triangular panels that can push out independently to propel the bot forward. But they also have a second purpose: Extended fully, they serve as a set of six legs. MorpHex’s wobbly roll shifts to precise steps that resemble a crustacean.

MorpHex is the product of one person. Norwegian engineer Kare Halvorsen built it as a hobby project, inspired by a lifelong love of robots. He built his first robot in 2006.

GroundBot

GroundBot’s sleek black exterior doesn’t say much, but the cameras in the two glass bubbles protruding from its sides make it clear: It’s watching you. GroundBot is built for surveillance in mud, sand and water at speeds of up to 6 miles per hour. It beams a livestream of everything its cameras take in and can be operated manually or autonomously. It is two feet tall and light enough to float on water.

Along with the two 360 degree cameras, GroundBot packs sensors that can detect radioactivity, heat, smoke, humidity, gas and narcotics.