Google’s plan to dominate the robotics field is well underway. Already, Google owns a company that make robot arms, a company that makes robot eyes, and now one that makes robot legs.

With last week’s acquisition of Boston Dynamics, an 80-person company that makes robots that can run faster than Usain Bolt and catch their balance after slipping on ice, the Mountain View giant has assembled a world-class team capable of developing the kinds of sophisticated robots we’ve only seen in science fiction. But what, exactly, is Google building?

Google has declined to elaborate on its robotics program, but various experts point to everything from off-road mapping to elder care.

"If you look at all the companies, each one is a key component to what a humanoid robotic system should be one day. But I really don’t know what they plan on doing," says Erin Rapacki, who worked for Industrial Perception, which makes 3D vision-guided robots, at the time it was bought by Google.

What, exactly, is Google building?

Colin Angle, the CEO of iRobot, is convinced that Google is working on home delivery: a robot drives up in a driverless car, then walks the package up to the door. Patrick Lin, director of the Ethics and Emerging Sciences group at California Polytechnic State University speculates the company is working on the first "social" robots, semi-autonomous machines that help people inside their homes or do jobs like law enforcement and public sanitation in the streets.

Google’s robotics acquisitions have had an emphasis on machines with joints, those that can work safely in proximity to humans, and those with a more traditional industrial approach, says Brian Gerkey, CEO of the Open Source Robotics Foundation. But as for what they’ll do with those capabilities? "Your guess is as good as mine," Gerkey says in an email. "Given Boston Dynamics’ focus on walking machines, perhaps they're going to build a legged robot? But what will it do?"

Boston Dynamics is Google’s eighth robotics acquisition in the last six months, and it got the most attention: partly because Boston Dynamics is a well-known name, and partly because it’s funded almost exclusively by the military. "Google Just Bought Some Really Creepy Military Robots," was a typical headline announcing the purchase.

Google says it’s not interested in military contracts, and a key role in defense robotics is one option for the company that we can likely rule out. With the typical Boston Dynamics military contract running between $2 million and $10 million, that payoff isn’t worth it for Google — at least, not yet. Furthermore, while Boston Dynamics is a world-class robotics company, it doesn’t own a ton of intellectual property — it’s more of a systems integrator, picking up existing technology and tying it together.

Experts point to everything from off-road mapping to elder care

It looks instead like Google is now using the same strategy for robots that it used for self-driving cars: picking up where military funding left off in order to refocus the technology on commercial applications. Google’s driverless-car team comes almost entirely from cherry-picking the roughly 200 teams that competed in the two Defense Advanced Research Projects Agency (DARPA) contests for a driverless car.

Now that all the foundational research has been done by startups and universities, often with military funding, Google is swooping in. The effort is being led by former Android operating system chief Andy Rubin, a known robotics buff. Rubin is a big-picture thinker, and he’s been obsessed with robots for decades. Angle recalls selling Rubin an iRobot B24 — a 2-foot-diameter research robot with three wheels and sonar sensors — in 1989 or 1990. "This is something that an individual would never buy," he says. "The only people who would buy it would be research universities and Andy Rubin."

While it’s possible that Rubin is simply buying up all the world-class robotics companies he likes, it seems more likely that Google is working toward a specific application. We won’t see evidence of that for several years, however.

There are still many technical challenges before Google can deploy robots on a commercial scale, like the fact that walking robots are huge energy hogs requiring bulky gas engines and are still way too expensive for consumers. Then there’s the fact that Google just bought eight different companies that all have different ideas about how to design a commercial robot, which will take time to reconcile. Last, there are the complications of programming a robot to interact safely with humans and complying with relevant regulations.

The foundational research has been done by startups and universities, often with military funding

That’s part of the reason why robot tech often ends up on the battlefield first, where safety precautions are less stringent. While Google says it doesn’t want to become a defense contractor, that doesn’t mean its robots won’t make their way to the military. Anything Google builds is likely to be adaptable for military applications. "It wouldn't take much to strap a weapons module onto these machines," says Dr. Lin.

But Boston Dynamics never wanted to be a military company, says Andrew String, a robotics engineer who worked at Boston Dynamics for five years. The military just turned out to be a convenient funding source — really the only possible funding source for that kind of research until Google came along.

"The fact that they were doing work on military contracts was kind of a sideline to the real effort, which was really developing better control algorithms for robots," he says. "It was kind of a byproduct of looking for applications. It’s not like the military came and said, ‘We need a legged sherpa.’ It was more like, Boston Dynamics said, ‘Hey, we’re really into legs, can you guys use these?’ And the military was like, ‘Yeah I guess.’"

With its foray into robotics, Google has created a new funding source for companies like Boston Dynamics that previously depended on military research grants. Google’s acquisitions will also likely inspire more venture-capital investment in robotics, which has historically been hard to find. Whatever the company turns out to be building, Google’s deeper pockets and its focus on commercial applications rather than military ones could have significant implications for the course of robotics development in the US.