The proposal is avidly supported by Angangueo’s local government and most of its residents, who would benefit most directly from the low-wage jobs the mine would bring — at first a few hundred jobs, and ultimately perhaps several thousand.

Elsewhere in the region, however, many are wary. Silvestre Chávez Sánchez, the elected leader of another community near the monarch reserve, told me, “We know that no mining project in Mexico has ever brought lasting development for local people, but has always had problems associated with natural resource destruction.”

Grupo México’s track record is not encouraging. In 2014, a huge copper mine it operates in the northern state of Sonora was the site of one of the worst environmental disasters in Mexican history. About 10 million gallons of toxic copper sulfate acid breached a dam at the mine and spilled into two rivers that supply water to more than 24,000 people.

In Angangueo, Grupo México wants to process up to 1,200 tons of ore daily, and says it will do so in an environmentally sensitive way, said María Isabel Ramírez, a geographer who studies monarchs at the National Autonomous University of Mexico in Morelia. But the company has been frustratingly vague on some key issues, including how much water and acid will be needed to extract copper, zinc, lead, silver and gold from all that ore, and where the resulting waste will be dumped. Nor has Grupo México fully explained where and how it plans to expand the old tunnel network that snakes beneath nearby mountains — the same mountains where monarchs roost every winter.

Ms. Ramírez worries that the huge volumes of water used by the mine will dry up mountain springs and threaten the viability of the oyamel fir trees where the butterflies roost. “We have many concerns about it,” she said, noting that the firs are already stressed by climate change and illegal logging, which persists despite years of efforts to stop it.

You might think that the World Wildlife Fund for Nature, which operates an active program within the reserve, would be leading the opposition. But the organization, which actively seeks corporate contributions, has been quiet so far, though the W.W.F.’s chief local representative gave me a blunt assessment. “My professional and personal position, W.W.F. aside, is that opening up the mine could have terrible implications ecologically and economically,” said Eduardo Rendón-Salinas, who heads W.W.F.-Mexico’s Monarch Butterfly Program.

The W.W.F. and other nonprofits, with some support from the Mexican government, are trying to develop alternate economic models for the region based on ecotourism, sustainable farming and logging, and native crafts, but funding has been limited and progress slow in a region where poverty is all but inescapable. The mine proposal, by contrast, offers a faster route — but to where, exactly? As Mr. Martínez told me: “It will provide work for a small group of people, but the cost may not be worth the benefit. We feel strongly that something like this may be catastrophic for the reserve.”

In Angangueo, memories are still fresh from the last catastrophe: the floods of 2010, when three days of heavy rain and hail produced mud slides and caused the local river to overrun its banks. After hours of gradual flooding, something suddenly gave way — no one is sure what — and sent a wall of water hurtling down the town’s main street. At least 30 people were killed, and hundreds were left homeless. Some locals blamed the honeycomb of mine tunnels above the town, but an official investigation absolved the mine and blamed the heavy rain — just another calamity for a community that has endured so much for so long.