Pyramids are some of the most enduring architectural achievements that humanity has created. But nothing lasts forever, and the Pyramid of the Sun in Mexico is no exception. The pyramid, a large tourist attraction, has held caches of artifacts, been excavated with robots and lasted well beyond other buildings on the site. But recent research suggests that this massive structure, built in the city of Teotihuican, is in danger of collapsing.

From New Scientist:

From 2010 to 2013, Arturo Menchaca of the National Autonomous University of Mexico (UNAM) in Mexico City and colleagues studied the interior of the pyramid using muons. These sub-atomic particles can pass through most materials, but are deflected when they hit denser ones. That means more muons reach the other side if an object has an internal cavity, filled with less dense air. So by tracking the paths of muons through the pyramid, Menchaca could create a 3D representation of its insides.

What they found was a giant pile of dirt. The 3 million tons of stone that make up the pyramid are supported by little more than soil, and one side of the soil is much less stable than the other. How long the pyramid has left is a question for future research and a lot of debate. So, if you'd like to visit, might want to start planning a trip.