The rain began on August 25, and it would fall, remarkably, for four more days. We know now that Hurricane Harvey dumped as much as 60 inches of rain over parts of Texas. Twenty trillion gallons in all. The equivalent of the entire Chesapeake Bay. Enough to push the Earth’s crust down two centimeters.

All of that water eventually had to go somewhere. It made its way to the Gulf of Mexico, and its volume was so massive that it did not immediately mix with the ocean. Nearly two months after the hurricane, a distinct blob of freshwater from Harvey is still moving through the Gulf. “We’ve literally never see that much freshwater added to the Gulf of Mexico at once,” says Kathryn Shamberger, an oceanographer at Texas A&M University.

Because it’s so unprecedented, scientists do not know exactly what effects the plume is having. Will it bring a path of destruction through marine ecosystems unused to freshwater? Will contaminants from land sweep through? Or will marine life quickly rebound? In the weeks after Harvey, scientists like Shamberger have mobilized to study the effects of this giant blob of freshwater. Shamberger and her colleagues transected the Gulf sampling water in late September, and they are embarking on a second research cruise this Friday to study the potential impacts on coral reefs.

What oceanographers do know about the interface of freshwater and ocean comes from studying rivers that naturally empty into the sea. The key is density. Because freshwater lacks dissolved salt, it is less dense and floats atop seawater. It becomes a barrier between the air and the ocean water, which can have nasty consequences. “The freshwater sitting on the salty water cuts off the oxygen from the atmosphere getting into the ocean, and then you get the dead zone,” says Steve DiMarco, one of Shamberger’s colleagues at Texas A&M.

Freshwater doesn’t move in one uniform blob, either. It can squirt and jet far away from the main body. Think of what happens when you add milk to your coffee, says DiMarco. In the first few seconds, the milk squirts through the coffee before fully mixing. Eventually, Harvey’s freshwater blob will mix into the ocean, too. DiMarco says he expects for it to take a few more weeks; the winter winds will stir it all up when they come in.

Meanwhile, a number of scientists have received or are expecting Rapid Response Research grants from the National Science Foundation to study Harvey’s freshwater pulse. Lisa Campbell, another oceanographer at Texas A&M, has received one such grant to look at the effects of Harvey’s freshwater on phytoplankton. “Previously when we seen slugs of freshwater, it changes the type of phytoplankton that grow,” says Campbell. And since phytoplankton are at the bottom of the food chain, that can have knock-on effects up the food chain from the copepods to the coral to the fish.

Satellite image on August 31 showing muddy floodwater pouring out of Galveston Bay (NASA)

Shamberger and DiMarco are going out to the Flower Garden Banks National Marine Sanctuary, a coral reef about 100 miles off the coast of Texas. Because winds have blown Harvey’s freshwater blob south and west, Shamberger says the corals have likely escaped the worst of it. But the reef may still be susceptible to the aforementioned squirts and jets of freshwater or more subtle changes in phytoplankton. In July 2016, recreational divers had noticed unusually hazy waters and dying corals in the Flower Garden Banks, which scientists suspect were the result of floodwaters that went through Houston during April. In the early days after Harvey, satellite pictures captured brown sediment-rich runoff pouring out of Galveston Bay. The team will also collect samples to test for contaminants.

“Often when we think about hurricanes and their impact on a coral reef, we are thinking about wave energy and storm surge and physical breakage of the 3-D reef structure,” says Adrienne Correa, a marine biologist at Rice University. But because Harvey dumped so much freshwater into the ocean, it might have this whole other set of effects, and this is a unique opportunity to study them. Correa is leaving on the cruise Friday as well, despite the fact that her home was flooded during Harvey and it’s still “pretty much a mess.”

DiMarco was actually on a research boat in the Gulf when Hurricane Harvey started threatening Texas. When the storm hit, they pulled into port west of Louisiana and started driving back to Texas through the rain. “It was just incredible how much water there was,” say DiMarco. “Every time we went over an overpass, the people in my car—and I was driving—were like, ‘I wonder if we’re going to come down in the middle of lake.’” That was when the planning of his post-Harvey research began. And now, nearly two months later, they’re still tracking the effects of that water through the Gulf.