Yury Uribe traced an invisible map of death in her neighborhood, pointing to house after house, nearly all of them touched by cancer, she said.

On that corner lived Doña Agustina, claimed by cancer in her stomach just like Coco, her neighbor across the street. Farther down was Eva, whose death from a ruptured throat tumor left her autistic son on his own. Colon cancer took Doña Juanita, who went to the hospital on a Friday and died the following Monday. And Estela lived over there until about two years ago, when lung cancer spread through her body.

By the end of the tour of her hometown, General Pedro María Anaya, Ms. Uribe had tallied an incomplete yet grim ledger: at least 42 cancer-related deaths, 15 survivors and six active cases, all clustered within a few blocks.

“It’s a lot of pain,” said Ms. Uribe, 45, a former seamstress who turned to activism after seeing her town besieged by unexplained illness. “It’s not normal to have this many sick people in a community”

For many residents of more than 20 communities surrounding a reservoir known as the Endhó Dam in Mexico’s Hidalgo State, cancer is just another symptom of a crisis decades in the making.

In the 1970s, the federal authorities quietly diverted Mexico City’s storm runoff and wastewater into the region, effectively transforming it into the dumping ground for more than 20 million people. Effluent from a heavy industry corridor — including more than 100 factories and an oil refinery fueling Mexico City and other states — has compounded the crisis, with the vast majority of those plants dumping their untreated waste into the Tula River, which empties into the Endhó.

The resulting stew has turned the reservoir into what residents call the world’s largest cesspool.

The United Nations special rapporteur on toxics and human rights has classified the Endhó basin as one of Mexico’s longest-running “sacrifice zones,” an area where local health and ecosystems are intentionally compromised to benefit the capital and its surrounding industries.

Sacrifice zones are a trade-off of modern urban planning. Areas where some people lose so the majority can thrive. The pattern spans the globe, from Louisiana’s “Cancer Alley” and Italy’s steel plants, to Canada’s oil sands and Mexico’s Endhó Dam.

Around the reservoir, the air is thick with sulfur and rot. White clouds of chemical foam float by homes and roads, while heavy organic waste in the water feeds an unstoppable carpet of aquatic weed.

“What we live through day after day is hell,” said Magnolia Sánchez, 51, a resident of San Mateo La Curva, another riparian community, whose cousin Maira died of breast cancer.

Older residents still remember when the polluted water arrived unannounced. Feliza Camacho was a newlywed in 1972 when she noticed the reservoir turning murky and smelly.

Then came the first victims.

“The lake was white with dead fish,” said Ms. Camacho, 73.

The decision to route Mexico City’s sewage lines and industrial runoff to the Endhó was initially seen as a win-win. As the capital grew, engineers desperate to shield the city from catastrophic flooding also realized the reservoir’s waters, loaded with organic nutrients from the sewage, could double as irrigation and a free fertilizer for the naturally arid valley.

Decades later, the massive influx of nutrients has sustained the region’s agricultural boom while also transforming the Endhó into an environmental dead zone.

Microbes have depleted the dam’s oxygen reserves. Aquatic life has vanished. In 2024, a thick carpet of invasive water hyacinth choked out the surface, growing over 27 times its initial size in just a few months, according to government estimates.

The labyrinth of crannies, nooks and holes within the tangled vegetation has turned the Endhó into a prolific mosquito factory.

When evening falls, people lock themselves inside. Cookouts and quinceañeras have been canceled because of the swarms. Families burn cardboard indoors, hoping the smoke scares off the insects, and purchase up to three cans of bug spray per week — a severe drain on their finances. Bedroom walls are speckled with people’s own dried blood, marks of their nightly battle with the creatures.

“We are living on edge. Being shut inside like this, we’re becoming aggressive. I see it in my own children, I see it in myself,” said Pablo Tinajero, 59, the community leader in Santa María Daxthó, a town by the dam.

At a nearby primary school, Daniela Álvarez, a teacher and principal, struggles to keep her students focused. Children constantly shift in their seats to shake off the mosquitoes, she said, and going to the bathroom, where the bugs congregate, has become a source of dread. Even Ms. Álvarez, 41, avoids pulling books from shelves for fear of releasing hidden insects.

“We’ve normalized the pollution, the bad air and the bad water,” she said. “But normalizing a plague is going too far. We shouldn’t have to teach our kids that this is normal.”

When Leticia Sánchez González’s right breast swelled last year, the pain was so intense that she said she had to quit her two jobs. Doctors told her not to worry, she said. It was likely an infection.

In June, new tests confirmed her suspicions: cancer. Her oncologist prescribed six rounds of chemotherapy to be followed by a mastectomy.

“So many people are getting sick now,” said Ms. Sánchez, 48, who lives near the dam in Santa Ana Ahuehuepan. “You have it, but so does the neighbor.”

The true extent of the illness is unclear.

According to raw data from Mexico’s statistics agency, cancer death rates in Tula de Allende — one of the two municipalities where the dam sits — have surpassed the national average every single year since 2016, at times spiking more than 50 percent higher than the rest of the country. In the neighboring municipality of Tepetitlán, mortality rates for kidney failure and urinary tract diseases have routinely dwarfed the national average, sometimes doubling it.

No public health data exist for the towns in the immediate vicinity of the dam, however.

The results of a census conducted by state and federal health brigades in 2024 were never disclosed. And the government has refused to fund epidemiological studies in the region, according to two researchers who spoke on the condition of anonymity for fear of losing public funding. Without that evidence, it remains close to impossible to know whether the surge in cancer cases is indeed caused by the pollutants in the Endhó, something else entirely or the result of random chance.

But to others, there is a clear connection between the Endhó’s polluted waters and the public health disaster. “The pattern is unequivocal,” said Juan Francisco Martínez, Hidalgo’s former director of epidemiological surveillance. “There is a correlation that is obvious, strong and coherent, even if there are no epidemiological studies to prove causality.”

Mexico’s Ministry of Health and the country’s health regulatory agency did not respond to a list of detailed questions.

As for residents, they say the proof they need is in their own backyards.

After Mexico City’s waste arrived, communities dug seven wells seeking clean water as the reservoir fouled. Yet for nearly 20 years, government studies have repeatedly confirmed that the wells — the main source of drinking water — are contaminated.

Arsenic, mercury and lead have all been found in the wells, violating maximum limits set by Mexican safety standards, according to reports by the country’s federal water authority seen by The Times. So have toxic levels of nitrates and fecal bacteria, as well as industrial solvents and byproducts of the petrochemical industry.

In 2024, an official report detected an identical chemical and molecular fingerprint shared between the reservoir and the local drinking supply, concluding that the Endhó’s toxic brew has seeped into the towns’ wells.

But Mayela Godínez, an official with Mexico’s water authority, dismissed that research as flawed, insisting that the agency’s technicians had assured her the well water “is not contaminated, that the quality is good.”