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Tucked away in rural western Maryland, past a dead-end road cut off by railroad tracks, a stream runs cool and clear over a ledge dotted with snails.

About the size of a flake of black pepper, one particular species of snail believed to live at this waterfall is playing a part in the legal battle over a massive immigration detention center planned for about 700 yards upstream in Williamsport, Maryland.

The fight between the state of Maryland and U.S. Immigration and Customs Enforcement could easily have been about federalism, due process, or human rights as ICE moved to house hundreds of people inside a windowless warehouse with little notice to the surrounding community.

Instead, it has proven to be about Appalachian springsnails—and a few other rare or endangered species, including freshwater mussels, fish, and crustaceans. These creatures may be surprisingly key to stopping a massive ICE detention warehouse from operating here in Maryland.

As ICE began buying up warehouses earlier this year as part of a $38 billion plan to expand its detention footprint, communities across the country have found varying ways to push back. In Social Circle, Georgia, the city manager shut off water to a warehouse ICE had purchased amid concerns about strains on local resources. In Hanover County, Virginia, the owner of a warehouse that ICE planned to buy backed out of the deal after local leaders and protesters opposed the plan.

Maryland didn’t have much chance to intervene before the U.S. Department of Homeland Security quietly bought an 825,000-square-foot warehouse in the state’s conservative western panhandle. State government also had little power to stop ICE from converting the warehouse into a detention and processing center: Local leaders came out in support of the project, and immigration matters fall squarely within the federal government’s authority.

With few cards available to play and grassroots opposition to the detention center growing, the Maryland Attorney General’s Office took a cue from the environmental justice movement, which has for decades used federal environmental laws to protect vulnerable communities from the unequal effects of pollution and development.

“Federal immigration........

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