Chicagoland Observatory

In this image, physicist Mike Crisler inspects the bubble chamber of the Chicagoland Observatory for Underground Particle Physics (COUPP) experiment. The experiment uses a glass jar filled with about a liter of iodotrifluoromethane, a fire-extinguishing liquid known as CF3I. When a subatomic particle hits a nucleus in an atom within the molecules of the fluid, it triggers the evaporation of a small amount of CF3I. The resulting bubble is initially too small to see but grows larger until researchers can take a photo of it and learn more about the nature of some of the smallest particles in the universe.

Credit: Fermilab