Scientists are building tunnels under South Dakota for a $3 billion experiment that could solve some of the universe's grandest mysteries

• Neutrinos are tiny particles that could hold secrets to some of the universe's biggest mysteries.
• The DUNE project hopes to learn more about these "ghost particles," which are difficult to study.
• To do so, the project will beam neutrinos about 800 miles between Illinois and South Dakota.

Nearly seven years ago, crews started hauling 800,000 tons of rock out of a former gold mine near Lead, South Dakota.

The three resulting, underground caverns are 500 feet long and almost tall enough to hold a seven-story building.