On September 1, 1991, a huge section of the 306-foot-long arch collapsed, leaving 180 tons of rock crumbled on the ground below.
"What caused this cataclysmic event? Water had been slowly shaping the arch for countless centuries, dissolving cement between sand grains, seeping into tiny cracks, freezing and expanding. What had finally upset the delicate balance," reads an information display at Arches. "Unseasonably heavy rains the preceding ten days may have filled pore spaces within the sandstone. The added weight may have finally overwhelmed the rock slab in its timeless struggle with gravity."