• Fourteen uranium cubes are what remain of Nazi Germany's nuclear arms effort.
• The Nazis had more than 1,000 of these cubes to start, but what happened to most of them remains a mystery.
• Researchers Tim Koeth and Miriam Hiebert have been tracking the history of these cubes. //

On someone's desk, one of the little gray cubes wouldn't raise an eyebrow. To the untrained eye, they look like paperweights.

"Marie Curie's granddaughter has one. She uses it as a doorstop," Miriam Hiebert, a historian and materials scientist, told Insider.

The weight of the 2-inch objects might be surprising, though — each is about 5 pounds. That's because they're made of the heaviest element on Earth: uranium.

The cubes were once part of experimental nuclear reactors the Nazis designed during World War II. As far as researchers know, only 14 cubes remain in the world, out of more than 1,000 used in Nazi Germany's experiments with nuclear weapons. Over 600 were captured and brought back to the US in the 40s. But even after that, what happened to most of the cubes is still unclear. //

Hiebert and Timothy Koeth, a professor of material science and engineering at the University of Maryland, are writing a book about the cubes. After years of research, they told Insider they think they know what happened.

Koeth describes the cubes as "the only living relic" of Nazi Germany's nuclear effort.

"They are the motivation for the entire Manhattan project," he said.