The signs of sickness were written across her face: a pale, queasy green and the inescapable smell of something rotten.

The milky green color that overwhelmed Baltimore’s harbor in late September was an ecological catastrophe unlike any local environmentalists recall. Known as a “pistachio tide,” the event occurred as changing temperatures flipped the waters, stirring up bacteria, depleting oxygen in the harbor almost completely and causing some of the largest fish kills in the city’s recent history.

Its triggers — dry weather and a sudden dip in temperature with the turn of the season — aren’t mysterious, but other questions are harder to answer. Why did this pistachio tide last so much longer than any before it? And how did this happen amid so much positive momentum for a cleaner harbor?

Even the harbor’s leading scientists remain stumped.

“As I’ve learned more and more, I realize that I don’t think we understand it well,” said Eric Schott, an expert in urban waterways who’s studied the harbor for close to 30 years.

Schott, an associate professor in the University of Maryland’s Institute of Marine and Environmental Technology, was speaking in a Fells Point conference room earlier this month. The annual Waterfront Partnership event meant to mark Baltimore’s progress toward a swimmable harbor had just concluded, and, outside the window, the pistachio tide’s drab waters lapped against a tall ship.

“I’ve had my simple-minded thoughts about what to do,” Schott said, “and I sort of don’t want to even suggest them.”

In the wake of this historic tide, experts like Schott have eyed the nearly $1 billion redevelopment plan for Harborplace as an opportunity to re-engineer the Inner Harbor against these environmental crises. Schott has applied for a $50,000 grant to bring together local scientists, advocates and developers to study the pistachio tide and possible solutions.

While thermal inversions like the one Baltimore’s harbor just experienced happen naturally, humans have made the consequences worse. Decades of industrial pollution, sewage and urban runoff have depleted oxygen and fed sulfur bacteria in the harbor’s depths, creating the conditions for ecological carnage when the water column turns over.

This particular pistachio tide lasted the better part of a month and drove a mass die-off of wildlife. Between late August and late September, the Maryland Department of the Environment says, 206,000 Atlantic menhaden died across three separate fish kills.

In total, the MDE estimates that 216,000 fish and over 400 blue crabs died in the harbor this season — figures some observers consider an undercount. According to state records, the harbor hasn’t experienced fish kills this deadly since the 1980s. A 1988 event left 100,000 fish dead and another in 1984 killed close to a million.

In short, the harbor experienced the ecological equivalent of a “heart attack,” said Adam Lindquist, vice president of the Waterfront Partnership, at the release of the group’s harbor health report card this month.

And “it’s going to take a big solution,” he said afterwards.

Lindquist is one of Baltimore’s most vocal advocates for a swimmable harbor. He’s keenly aware that, while pistachio tides aren’t harmful to humans, dead fish and a stinking harbor do little favor for his movement.

Some measures for improving the harbor’s ecosystem are already incorporated into the vision of MCB Real Estate, the local developer tasked with the expensive job of reimagining the downtown waterfront. The developer recently met with the Waterfront Partnership’s leadership to discuss ideas for stemming the pistachio tide.

MCB’s roughly $900 million vision, recently approved by a voter referendum, would constitute the most consequential change for the downtown waterfront in half a century.

The plan includes raising the promenades to protect against sea level rise, expanding green space and planting 500 new trees around the harbor to provide cooling and absorb runoff. In the water, the developer wants to add roughly 36,000 square feet of wetlands, expanding on the micro-ecosystem outside the harbor’s National Aquarium.

Renderings of MCB’s plans feature people swimming. (These images have drawn criticism for fantastical-seeming ideas; they also show cable cars crossing above the harbor).

The Waterfront Partnership, meanwhile, is exploring the possibility for a permanent swimming area in the harbor.

Baltimore’s most fervent water advocates acknowledge that the harbor will never resemble its form a few hundred years ago, before Europeans arrived, when the Patapsco River was lined with marshes and its shallow waters offered a vibrant home to crab, menhaden and oysters.

Over the centuries, Baltimore’s harbor was industrialized, dredged and heavily polluted. Even its borders have changed: The harbor once extended north all the way to Water Street, an extra three city blocks, but the city filled in that area with rubble after the Great Fire of 1904.

The city removed the harbor’s natural shorelines and “put it in a vertical box,” said Charmaine Dahlenburg, director of field conservation for the National Aquarium, which tracked the harbor’s oxygen levels during the pistachio tide.

“The harbor became the way it is over 200 years,” she said, “and we’re not going to clean it up in two years.”

Dahlenburg remembers a major pistachio tide in 2016 as the first time scientists measured the consequences for oxygen. It’s not clear whether other cities suffer from the same problem, but she pointed to the aquarium’s new floating wetland as a potential model for a more resilient harbor. When the pistachio tide tanked oxygen across the ecosystem, the wetland provided a safe area for fish.

Harbor advocates have mulled other creative fixes, too.

A pair of boats tool around the Thames in London, finding oxygen-starved areas to inject with oxygen, an intriguing option to Chris Streb, an environmental engineer in Baltimore with the firm BioHabitats.

Others like Ryan Woodland, a fish ecologist with the University of Maryland Center for Environmental Science, pointed to engineering techniques like Sydney’s “living seawalls.” Though the Australian city has an urbanized oceanfront, its bulkheads are equipped with barnacle and boulder-shaped formations that help aquatic life take root on otherwise sterile walls.

Another of Schott’s “simple-minded” ideas: Could Baltimore make the harbor shallower?

Since it doubles as a transportation hub, the harbor is dozens of feet deeper today than it once was, which Schott thinks may help explain why pistachio tides can be so devastating. Deep channels have more room for oxygen-deprived areas at the bottom, so they deal that much more damage when cool weather inverts the waters.

Yet no one really can say why this year’s pistachio tide was so bad.

“It just seems wild to me that the year is 2025, and we still have big, fundamental questions about the Baltimore harbor,” said Alice Volpitta, a water quality watchdog for the group Blue Water Baltimore.

Funding for water quality monitoring in the harbor remains too short, Volpitta said, but she’s glad to see more attention and resources going into understanding the problem. She believes it’s possible to restore the harbor for fish and other wildlife.

Still, Volpitta added, “it’s going to take a lot of money and a lot of political will that probably doesn’t currently exist.”

It’s not lost on Adam Genn, vice president of MCB’s redevelopment team, that a green, putrid harbor filled with dead fish isn’t good for the community or for business.

MCB is open to new eco-friendly suggestions for its redevelopment, Genn said. He noted that other groups could take steps, like adding more floating wetlands, without the developer’s involvement. Even making parts of the harbor shallower may be on the table, he said, as long as it doesn’t interfere with tall ships and other water transport.

Man-made infrastructure created this problem, and Genn is convinced that man-made infrastructure can solve it, too.