Although an Augustóbriga aqueduct, paved roads and thermal baths were sacrificed to the dam, a second-century A.D. temple, known as Los Mármoles (the Marbles), was dismantled stone by stone and reassembled on higher ground four miles away. The 20th-century inhabitants of the settlement were relocated, too.
Since resurfacing in the summer of 2019, the Dolmen of Guadalperal has re-emerged every July, only to be swallowed by the lake again every September. Mr. Castaño and his organization, Raíces de Peraleda (the Roots of Peraleda), support a petition drive to have the government move the cracked and toppling megaliths to a new location on permanently dry land. “The combination of climate change and new electricity policies is very bad news for the preservation of the dolmen, since changes in the environment rapidly weaken the stones,” Mr. Castaño said.
The exposed monument is also imperiled by tourists, many wielding smartphones and selfie sticks, who reach the site on boats operated by a private firm. “Human presence on the site is very detrimental,” Dr. Bueno said. “In our last measurements we verified that the constant trampling of visitors from 2019 to 2021 caused a decrease in the sediments, due to their plasticity, which left the dolmen’s supports almost without a base.”
With so much unmonitored traffic at the monument, Mr. Castaño is concerned about the possible damage to the fingerlike menhir at the entrance to the burial vault. The slab is engraved with a vaguely anthropomorphic shape on one side and a squiggle on the other. Mr. Castaño believes that the squiggle depicts the contours of the Tagus before the dam was erected. A crook in the squiggle, he said, corresponds to a “strange bend” in old maps of the river. “If the curvy line does represent the ancient course of the Tagus, the menhir may be the oldest realistic map in the world,” he said.
Dr. Bueno demurred. “The hypothesis of a map is based on a pareidolia,” she said, meaning the tendency for perception to impose a meaningful interpretation on an ambiguous visual pattern. Dr. Bueno noted that the squiggle is geometric and similar to twisty markings found in megalithic art across Europe.
Her conclusion: It’s a snake.
