It’s not hard to find the Gowanus Canal on a hot day in Carroll Gardens or Park Slope. Just follow your nose, and the tangy miasma of raw sewage and harbor brine will lead you straight to its oil-slicked shores.

It is one of the nation’s most polluted bodies of water, but beneath the rainbow film of oil, the carcasses of errant marine life and the “black mayonnaise” of congealed coal tar, the Gowanus Canal may be holding some of the city’s oldest secrets.

Now, as the EPA dredges the fetid waterway amid a $506 million federal Superfund cleanup, workers are unearthing relics from Brooklyn’s industrial heyday to its hipster-haven present — including a World War II-era “crash boat,” massive wooden textile spools and wagon wheels from when Brooklyn was its own city.

And that’s just a sampling of the archaeological finds from one small section of the 1.8-mile-long canal called the Fourth Street Turning Basin, where contractors are wrapping up a “pilot” dredging study before they begin a broader cleanup of the rest of the Gowanus.

The big problem, according to archaeologists hired to assess the dragged-up detritus, is cleaning off the grime-encrusted treasures.

“When this stuff first comes out, it kind of has a . . . fecal-petroleum-musty smell that you have to have a good stomach to be around,” said Jonathan Bream, an archaeologist with Archaeology & Historic Resource Services. “The grossest part of it . . . is when something accidentally falls in the drink and splashes you, because you’re not expecting it.”

The researcher wears a Tyvek suit and protective gear to keep splash-back at bay as he power-washes the canal’s sludge — once found to harbor gonorrhea — from the workers’ discoveries.

Since the Industrial Revolution, textile manufacturers, gas refineries and coal shippers all dumped their toxic waste directly into the canal, leaving behind a festering cocktail of poison slime composed of polycyclic aromatic hydrocarbons, polychlorinated biphenyls and heavy metals — including mercury, lead and copper — to fester at the creek bottom.

EPA workers are dredging the canal and properly disposing of the toxic sluice before they “cap” the creek bed with a layer of clean sand. They’re also restoring bulkheads so contaminated earth along the shores cannot come loose and slide into the remediated water.

Taxpayers will foot most of the bill, but the feds got National Grid to kick in $100 million of the $506 million price tag, because the company is the successor to the manufactured-gas plants that lent to the canal’s contamination.

But workers aren’t just pulling up dirt — and Bream’s job is to determine whether anything dragged up has historical value. Then the EPA and the city have to figure out what to do with it.

The biggest haul by far is the crash boat, which was built in Miami and later reincarnated as a Fire Island ferry, a Bronx houseboat, a floating Brooklyn art space and finally an LGBT party boat named the SS Gay before it was finally scuttled in the soupy canal.

The 63-foot-long, wood-hulled Aircraft Rescue Boat was constructed by the Miami Shipbuilding Corp. in 1943. With four engines turning out a combined 2,000 horsepower, it had one mission: zip out to downed airplanes at 50 mph and scoop up pilots before the sharks got to them.

It survived World War II and the Korean War and retired to a life of leisure in 1963, when it was purchased by the Point O’Woods Association and turned into a Fire Island ferry.

Renamed the Point O’Woods V, it shuttled beachgoers across the Great South Bay, managing to eke out one more rescue mission while deployed on the island.

“Notably, this ferry assisted in evacuating people from Fire Island in advance of Hurricane Belle in 1976,” AHRS reports.

The Point O’Woods V was retired again in 1985, when its mahogany and cedar hull was deemed unfit for ferry service.

By 1989, it was named the ­Kokkomokko and served as a houseboat on Westchester Creek in the Bronx. A husband and wife lived on the boat while she went to law school and he battled an illness, according to Francois Guillet, the former owner of the Metro Marine marina where the boat was docked.

“They liked the low cost of living,” Guillet told The Post. “They were only paying $400 a month.”

The boat’s story nearly ended in Westchester Creek when a chunk of ice ripped through its hull in 2003, sinking the craft. The couple bailed, but Guillet raised and repaired the wreck himself.

He sold the patched-up ship to an artists’ collective for $1 in 2005.

“I found somebody who would take it as is,” he said. “I gave it to them for a dollar because the boat was in bad shape. I towed it for free to the Gowanus. Once I delivered the boat, I showed the people around and that was it. We were both happy.”

At the time, it was the first houseboat to permanently dock on the waterway, researchers noted.

The artists renamed it The Empty Vessel Project and used it as a public art space. But by 2006, maintaining the floating relic became too unwieldy and the artists sold it — again for just $1 — to a group that transformed it into an LGBT party boat officially dubbed the Green Anchor Yacht, but more commonly referred to as the SS Gay.

“That boat was a pretty amazing space,” said artist Duke Riley, who spent time aboard it when it was the Empty Vessel Project and the SS Gay. “Both groups sort of used it as a community space, and it was sort of like a wide-open cabin down below that was pretty spacious.

“It was a pretty cool moment in that time of Brooklyn — people living on the edge.”

Neither researchers nor Riley know exactly how the SS Gay ended up at the bottom of the canal. Some say it was set ablaze during a party. Riley produced an intaglio print dated July 25, 2009, and titled “The Scuttling of the S.S. Gay,” which depicts several people sinking the boat, but said the representation was fictional.

Despite the storied history in New York waterways, the boat’s historical value as a World War II relic is practically nil because of all the renovation it’s undergone.

“In its current wrecked and deteriorated state, the boat lacks integrity as an example of a World War II era ‘crash boat,’ ” researchers wrote in a report to the EPA.

“The removal of the original superstructure at the time of its conversion to a ferry and the later removal of its engines and machinery further weaken the boat’s historic integrity,” they added.

“As a result, the wreck has no archaeological value, nor is it eligible for listing in the National Register of Historic Places.”

The boat represents just a slice of the canal’s more recent history, but workers have also dredged up older relics.

A gantry crane liberated from the canal was likely loading horse-drawn delivery carts in the late 1800s before the consolidation of New York City in 1898, according to Bream.

“It would have picked up the coal from the barges on the canal and it would have moved it to the coal ‘pockets’ — the coal went in those and then they drove it in a horse-operated wagon or a gasoline-powered truck,” he said.

Some large spools and skeins that have been dredged up appear to have come from the Zobel Color Works, a textile and dye manufacturer in the early 1900s.

The only personal item found is a ceramic kettle, according to Bream.

“It adds a little bit of personality to the canal,” he said. “You know that a person was actually here, drinking coffee or tea or something that.”

What’s left is to figure out what to do with all this stuff. Much of it will wind up in the garbage heap once it’s cleaned off. But if anything found is valuable, it’s unclear who’d get to hang on to it.

“Because it’s a city water body, it’s a little bit unclear as to the ownership,” said EPA spokesman Elias Rodriguez.

“EPA doesn’t own this material — it’s something that came out of a city waterbody.

“The final disposition of what’s coming out of the canal is yet to be determined.”

Next, the EPA will begin dredging beyond the Fourth Street Turning Basin, with a target date for ending the entire remediation somewhere north of 2022.

Bream is hoping the ensuing work dredges up even older parts of Brooklyn’s history.

The canal played an integral role in the American Revolution, when an American regiment led by Alexander Stirling beat a hasty retreat across the then-marshy swamp after tides turned during the ill-fated Battle of Brooklyn — but archaeologists will have to wait until they dig farther into the canal to uncover any Revolutionary War relics, because the basin is manmade and relatively new.

“It would be nice if we could find something, say, from that Battle of Brooklyn or, say, from the Native Americans that were here,” Bream said.

“But knowing how this turn basin was built, and knowing how it was built in the 1860s and 1870s, there’s a high probability that those objects won’t exist.”

Of course, there are other mysteries the canal may solve — such as the disappearance of a certain missing mobster — but workers haven’t made any startling discoveries yet.

“We have found no bodies, we have found no gold,” Bream said. “We have not found Jimmy Hoffa nor any of his relatives.”