In a small Key Largo neighborhood, the tide came in — and didn't go out for almost a month.

Residents sloshed through more than a foot of saltwater that lapped at their front yards, knocked over their trash cans, created a mosquito breeding ground and made their roads nearly impassable. Some residents rented SUVs to protect their own cars. Others were homebound.

One started a Facebook page to document the flooding: Key Largo Community Swamp.

That was the fall of 2015, courtesy of freak weather and high tides. Neighbors have clamored for solutions since, and Monroe County has finally pitched a potential fix. Officials want to elevate the lowest, most flood-prone road in the Twin Lakes Community of Key Largo and in the low-lying Sands neighborhood of Big Pine Key, 70 miles south — and 2018 might be the year it happens. The county will start accepting design proposals in the coming weeks, and money for construction could be available in October.

It's a small but significant project — it will be the first road project in the Keys specifically designed for adaptation to future sea level rise, a clear and present problem for the famous chain of islands. The county has already spent $10 million on road projects that include elevation, and plans to spend at least $7 million more in the near future. But these are the first to include collecting, pumping and treating the stormwater that runs off the newly raised road.

These small stretches of road are test cases for the county. Monroe hopes to use lessons learned here on the rest of the roads that climate change will swamp in the years to come. Out of 300 total miles of county roads, half are susceptible to sea level rise in the next 20 years, said Rhonda Haag, the county's sustainability program manager.

It won't be cheap. Early estimates show that raising just one-third of a mile of road above sea level could cost a million dollars in Key Largo and more than $2.5 million in Big Pine.

For comparison, the county spent $3.3 million repairing about two miles of road in Lake Surprise Estates in Key Largo. That was a less ambitious fix that rebuilt some parts, elevated others and included more limited drainage additions.

This new project is nothing like the county has done before, said Haag. It's "like comparing oranges to apples."

Elevating is pricier than repairing a regular chunk of road because the process entails much more than just pouring extra asphalt on top. All that water from incoming tides has to go somewhere, and handling it requires building new structures like pumps and pipes.

"In the Keys we don't have drainage infrastructure," said Haag. "Basically it's blacktop on the road."

The unique geography of the Keys plays a big part in why drainage solutions common in other areas won't work on the South Florida islands. Underneath the dirt on each island is porous limestone rock. Sometimes, when water levels get high, that sponge-like rock is filled with groundwater. It can degrade the materials used to form the base of the roads and crack asphalt.

High groundwater also means engineers can't count on the ground to absorb runoff, so they have to turn to pumps to send the water elsewhere, as Miami Beach does. The city has spent more than $125 million on a drainage system and elevated roads.

Unlike Miami Beach, Monroe wants to clean the water they're pumping back into the ocean. Miami Beach filters the water for large objects such as plastic bottles, but studies have shown the pumps pick up fecal matter from the roads and wash the pollution straight into the bay.

"You cannot do that," Haag said. "We absolutely, positively have to treat the stormwater before it's released."

Treatment is pricey, and building the necessary pipes and treatment plant will likely make up a significant amount of the project's costs. Pumps also require backup generators in case of emergencies, which require extra land, something the current cost estimate doesn't factor in.

When the county first considered raising the roads, the plan was to use gas taxes and road repair money. Ironically, after Hurricane Irma decimated a large swath of the lower and middle Keys (including the project area in Big Pine) the county now has access to FEMA hazard mitigation grants. Haag said Monroe County is considering applying.

An estimate shows raising less than a mile of road could cost the county upwards of $3.5 million. In Big Pine, that chunk of road is going up a foot. In Key Largo, it's being raised six inches. At best, that's a minimalist approach. Those elevations are just an inch above what researchers say is necessary for both roads to be above sea level for the next 25 years.

To elevate any higher, "Well, that's expensive," Haag said.

Bringing the Key Largo neighborhood up to a foot above sea level would double the length of road included and quadruple the price. In Big Pine, elevating to 18 inches above sea level would require spending almost $9 million on 1.29 miles of road.

Still, researchers say the fixes would buy some time for the low-lying islands. The roads are expected to be higher than sea level until at least 2040. By then, scientists predict climate change will raise the sea level 7 to 18 inches, which would leave the roads — if left as they are — underwater.

Even with these millions of dollars in reconstruction, there isn't a guarantee that these roads will stay dry. Haag said the new roads should still see an average of seven days of flooding a year, depending on which sea level rise prediction comes true.

"To keep everyone dry all the time dramatically raised the price," she said. "You'd have to go much higher."