HASANKEYF, Turkey – There was something about Hasankeyf that made visitors fall in love with the town on first sight.

Graced with mosques and shrines, it lay nestled beneath great sandstone cliffs on the banks of the Tigris River. Gardens were filled with figs and pomegranates.

The golden cliffs, honeycombed with caves, are thought to have been used in Neolithic times. An ancient fortress marked what was once the edge of the Roman Empire. The ruins of a medieval bridge recalled when the town was a trading center on the Silk Road.

Now it is all lost forever, submerged beneath the rising waters of the Ilisu Dam, the latest of President Recep Tayyip Erdogan's megaprojects, which flooded 100 miles of the Tigris River and its tributaries. The steadily expanding reservoir displaced more than 70,000 inhabitants. Unexplored archaeological riches were swallowed up along with farms and homes.

The waters have rendered Hasankeyf an irretrievable relic of the bygone civilizations that had been drawn to the valley, carved over millennia by one of the Middle East's greatest rivers.

When Erdogan turned on the first turbine of the hydroelectric dam in May, he promised that it would bring peace and prosperity to southeastern Turkey. The dam would contribute billions to the economy and irrigate thousands of hectares of farmland, he said.

Officials have emphasized that hydropower offered their greenest option when they decided to push ahead with the dam a dozen years ago, allowing Turkey to reduce its dependence on imported coal and gas.

But many who lost their homes and livelihoods are bitter and traumatized. Environmentalists and archaeologists are frustrated, too, at the loss of the valley and its treasures.

Their efforts to save Hasankeyf collapsed amid Erdogan's increasing authoritarianism. International law, lagging the shifting attitudes around climate change and the value of protecting the environment, was inadequate for safeguarding the cultural heritage, they say.

Zeynep Ahunbay, a conservation architect, campaigned for more than a decade to save Hasankeyf. "To disturb or change the natural process of the river is also criminal," she said. "You lose the beauty, you lose history, you lose nature. You are a loser all the time."

When Erdogan first announced his determination to build the dam, he championed it not only for the energy it would provide Turkey's expanding economy but also for the development it would bring to the impoverished and insurgency-riven southeast.

Money seemed no object. The government built two towns to relocate those displaced, and highways and bridges to skirt the reservoir. Companies closely allied to Erdogan's government won the contracts.

The project had become a moneymaking exercise, a bureaucrat said, asking that he not be identified by name for fear of reprisals.

"They spent a horrendous amount of money," said activist Emin Bulut, who said the bill ran to trillions of lira. "They could have fixed all the problems of the south with that."

In 2012, government officials arrived to begin evaluating property to compensate those who would be displaced. But the money became a source of resentment, dividing the community, and even families, and raising accusations of corruption. It broke apart any unified opposition to the dam.

"We surrendered when they came to measure the houses," said Birsen Argun, 44, who together with her husband ran the Hasbahce Hotel, the only hotel in Hasankeyf.

Her husband tried to persuade his brothers to refuse the money, but they accepted the payout. People withdrew the money from accounts without telling others, she added.

Many of those who tried to organize a protest movement grew up in Hasankeyf and were even born in the cave homes of the citadel, like Arif Ayhan, 44. Politics split the campaign, he said, especially when supporters of the outlawed Kurdish movement, the Kurdistan Workers' Party, joined the rallies against the dam, enraging police.

"This is why we failed," he said. "We live in the most beautiful place in the world, but we could not appreciate the value of this place."

In the town of Temelli, perched above the dam, Hezni Aksu, 60, looked down from his terrace to where his family's farmhouse and lands were among the first to go under water.

"This land was from our ancestors," he said bitterly. His son was now unemployed. "They made migrants of us."