Some fear the breach—now roughly the width of two football fields laid end-to-end—will magnify the power of the next superstorm, allowing more water to fill the bay and crash along the coast. “It’s a giant hole,” said Aram Terchunian, a Long Island coastal geologist who has worked as a consultant on other breach-closure projects. “What do you think is going to happen? You’re going to get a storm surge, water’s going to come flooding in through the inlet and it’s going to fill up the Great South Bay. It’s not rocket science.”

Others see a positive development and symbol of nature’s power of self-renewal—an instance of the ocean breaking through a barrier of land to rescue a bay that overfishing and overdevelopment had rendered all but unrecognizable. The blueness of the water around the inlet forms a stark line against the brown tide, reminding some residents of just how far Long Island’s waterways had fallen.

“The bill came due in a lot of ways,” Marshall Brown said, “with Sandy and what it uncovered.”

Brown remembers the Great South Bay before the brown tide came. He was a kid growing up in Sayville, a Long Island hamlet that sits at the water’s edge. Native eelgrass grew so thick, he said, that boaters would have to reverse their outboard motors to spin slimy strands off their propellers. In the mid-1970s, when Brown was a teenager, baymen raked and dredged more than half the hard clams eaten in the country off the bay’s floor. Black-and-white photographs from that era show clam boats stretching to the horizon.

When Brown returned to Sayville for a high school reunion 35 years after he left for college, he walked his son down to the beach and found something entirely different. The water looked “disgusting,” he said —“dark, dirty, lifeless.” Fields of eelgrass were gone. So were the baymen. Just a handful still eked out a living off what was left on the bay’s floor.

What happened in between is one of Long Island’s most storied environmental collapses. Scientists blame, in part, overharvesting by the shellfish industry. But blame has fallen increasingly on the brown tide, which blocks sunlight and kills the eelgrass beds that shellfish use as nurseries. It also causes shellfish to close up and stop feeding, although scientists don’t know exactly why.

In recent years, studies have traced the brown tide to nitrogen pollution flowing from the island’s buried backyard septic systems. Long Island is home to 2.8 million people and part of the most populous metropolitan area in the country, but huge swaths of it aren’t connected to sewers, relying instead on septic tanks that allow wastewater to collect underground and leach into the earth. From there, nitrogen—a nutrient found in human waste—winds its way through the groundwater and into the bays, where it feeds the algal blooms.