Philip Metzger has been playing with mud. Experimenting, you could say, as he’s a planetary scientist at the University of Central Florida and co-founder of NASA’s Swamp Works lab. In any case, his lab has been stuffing Martian clay into cupcake decorating bags and extruding it into what Metzger himself admits sometimes look like an “animal dropping.”

But one man’s cupcake decorating with mud is another man’s prototyping of 3D-printing on Mars. And 3D-printing could solve the single biggest hurdle to a crewed Mars expedition: the cost of transporting everything humans need to survive on the red planet.

It’s a mass problem. The more mass you have to take, the more expensive it is to escape Earth’s gravity and get to Mars. And some of the heaviest cargo will be material to shelter astronauts from the radiation zipping through Mars’ thin atmosphere. With 3D-printing, you don’t need to bring shelter. You build it out of dirt or ice already on Mars.

NASA is all aboard the 3D-printing train. Last year, it unveiled winners of its first 3D-printed Mars habitat design challenge, and the architectural renders of the winning entries were all sleek and futuristic, as renders of unbuilt buildings always are . In reality, the current state of the art for Martian 3D-printing looks more like the clay logs Metzger has been documenting on Twitter.

Martian (sim) clay logs. Top: still damp. Bottom: vacuum dried. I'm surprised the 50% mix crumbled so badly. Need to repeat w/ more samples. pic.twitter.com/H9Sug4TB09 — Dr. Phil Metzger (@DrPhiltill) October 13, 2016

If the technology looks low-tech, it’s deliberate. “We’re rethinking how to do space technology by taking cues from less developed parts of world,” says Metzger. The logic goes like this: If a valve breaks in a complex machine on Mars, an astronaut can’t go online to order a replacement with next day delivery. (It’s more like nine months, assuming Mars and Earth are in their most favorable alignment.) So the idea is to start simple and slowly build up technological capabilities: clay to metal to plastic to electronic equipment. Eventually, Mars will have the refineries and factories to make complicated machines itself. This is “bootstrapping,” and it’s Metzger’s vision for space exploration.