South Floridians are bracing for Hurricane Irma’s potentially catastrophic damage. Weather advisories are warning of massive flooding, which will likely render roads impassable and homes uninhabitable.

They also face a less visible, yet frightening, potential consequence: contamination from uncontained poop.

Just as it downs electricity poles and submerges streets above ground, the avalanche of water unleashed by a hurricane disrupts the order of things down below, where waste goes after you flush. South Florida’s sewer infrastructure is particularly vulnerable. Like many urban areas across the US, its wastewater lines are rickety. On top of that, many locals store their sewage in underground septic tanks, whose contents are prone to escape during storms.

The prospect of poop-laden water pooling around in the streets of Miami is scary enough, yet it’s just a symptom of a much bigger problem that plagues hurricane-prone Florida. Rising sea levels are upending its ability to deal with floodwater—and both sea levels and flood-inducing storms will get worse with climate change.

That’s the real horror story.

Draining the swamp

The only reason the naturally swampy terrain of South Florida can sustain more than six million people today is because its previous residents dredged and drained it. The operations started in the late 1800s, and by the 1970s Floridians had built an expansive network of canals, levees, and pumping stations to keep water at bay. The system, which was designed to let gravity drag groundwater downstream to the ocean, was based on 1930s sea levels, as Frederick Bloetscher, a water-management expert, pointed out during a 2014 US Senate hearing on Florida’s changing coastline.

Fast forward nearly 90 years, and sea levels are higher. The rise “kind of frustrates that initial goal, and as a result we see more frequent flooding not only on the coast but inland, because inland doesn’t discharge as easily,” Bloetscher told Congress.

The swelling oceans are also complicating draining by seeping into Florida’s porous limestone floor and raising groundwater levels as well. The water coming in from below is also drenching the soil, reducing its ability to collect water coming from above.

So in some parts of Florida, such as Miami Beach, the barrier island dangling off the Miami coast, flooding has become a near daily part of life.

Mixed waters

Backed up toilets are also becoming a more common occurrence. The waste produced by about a third of the people going to the bathroom any given day in Florida (that includes tourists) goes into a septic tank. In order for a tank to do its job, there needs to be room for the liquid portion of the waste to slowly filter down into the ground. When groundwater levels go up, though, they push the waste back up, sometimes resulting in a poop flood.

The area’s creaky sewer system is another potential source of fecal contamination. It was in such bad shape that the US Environmental Protection Agency sued Miami-Dade County in 2012 for violating various water pollution laws. The county made notable improvements last year, reducing the volume of spilled sewage by 55%, or roughly 1.5 million gallons, according to its 2016 annual report to federal regulators. But a single spill this year, amounted to half of that. To Kelly Cox, staff attorney at Miami Waterkeeper, a clean-water advocacy group, that’s evidence that “the water infrastructure is at risk and could be easily compromised by a storm the size of Irma.”

But leaky pipes could be a minor problem compared to the flooding of one of Miami-Dade’s three water treatment plants, two of which are vulnerable because they lie low-coastal areas, says Cox.

As Hurricane Sandy showed, that can have some nasty consequences. During that storm, the surging ocean dumped nine feet of water on the pumping system engines of the Bay Park treatment plant, in Nassau County, NY. The resulting raw sewage backlog made its way into channels, streets, and people’s houses homes, according to Arcadis, the international engineering firm that helped repair the plant.

On top of the pollution, Sandy put the plant out of commission for two weeks. So even as the floodwater receded, people still couldn’t flush their toilets. It’s a scenario that could unfold in Florida in the aftermath of Irma, says Edgar Westerhof, Arcadis’s national director for flood risk and resiliency.

Pump it out

Waste appears to filtering into the floodwater even without a single drop of drain. In Miami Beach, an exceptionally high, or king, tide is enough to cause a flood. To deal with the water rush, the city has been installing an expensive system of pumps. That gets the problem out of the streets, and dumps it straight into the ocean.

Sewage contamination in water leads to algal blooms, which can have a host of dire consequences that threaten humans, wildlife, and Florida’s multi-billion dollar tourism industry.

The water that Miami Beach pumped into Biscayne Bay in 2014 and 2015 after king tides had live fecal bacteria that significantly exceeded regulatory limits, according to a study by researchers from the US National Oceanic and Atmospheric Agency and several Florida Universities. Much of it came from humans. “This most likely comes from tidal floodwater flushing of old and leaking underground sewage and septic infrastructure in the interior of the island,” the authors concluded.

Local authorities dismissed the study’s results, calling them sloppy science. It will be more difficult to dismiss whatever evidence about Florida’s vulnerable sewer system Irma washes up.