Darwin’s Arch, an iconic rock formation that for years was a much-photographed destination for tourists visiting the Galápagos Islands, is no longer an arch.

Two rock pillars were left standing after the bridge-like portion of the arch crumbled into the Pacific Ocean on Monday. The collapse was the “consequence of natural erosion,” Ecuador’s Environmental Ministry said.

A Galápagos tour company, Aggressor Adventures, said that a group of its guests had “experienced a once in a lifetime event” on Monday morning when the arch “collapsed in front of their eyes.”

Some locals who work in the tourism and dive industry have now nicknamed the remaining rock towers the “Pillars of Evolution,” the company said — a reference to the fact that Darwin’s Arch and nearby Darwin Island are named after naturalist Charles Darwin, whose observations of the area’s diverse wildlife formed a foundation for his theory of evolution by means of natural selection.

The arch previously measured roughly 141 feet high and was 230 feet long, according to Reuters. Shaped over the years by the wind and waves, it sat on an underwater plateau that is popular with scuba divers. It could be reached only by boat, and landings were strictly prohibited.

According to Ecuadoran newspaper El Universo, scientists believe that the rock formation was originally part of Darwin Island, which is located only about a half-mile away. The Galápagos are home to thousands of underwater volcanoes, and seismic activity may have played a major role in shaping the unusual landscape.

“The beauty of nature lies not in its permanence, but in its constant transformation,” the Galapagos Conservancy said in a statement. “Galapagos, more than any other place on Earth, is a symbol of evolution and change, so although we are saddened to lose this iconic structure due to natural erosion, it is a reminder of the power of nature’s architecture and the need to preserve wild places while we still can.”

The island archipelago is deemed a UNESCO World Heritage Site because of its marine biodiversity. It draws roughly 170,000 tourists a year while maintaining visitor quotas and keeping some islands, including Darwin Island, off-limits to the public.

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