An area the size of Chicago in Yellowstone National Park has been inflating and deflating by several inches over the past decade

The Norris Geyser Basin, the oldest, hottest and most dynamic thermal area in the park, was observed to rise 5.9 inches each year from 2013 to 2015 - an unusual event that left researchers baffled.

Now, using satellite radar and GPS data, experts have determined the ground deformation was caused by magma intrusions trapped below the basin's surface.

As magma made its way to the surface, the pressure pushed rocks above it up and created an erratic pulsating effect, according to National Geographic.