Jessi Halligan has spent 240 hours of her life sifting through muck in the pitch black dark thirty feet under the surface of a Florida river. Halligan, an underwater archeologist working out of Florida State University, loves her work but says she sometimes feels like she’s commuting to a sensory deprivation chamber. It’s an apt description for work that is exhilarating, tedious, and necessary to the progress of archeology as a discipline. If Halligan is right, archeologists cannot understand how North America was initially settled without swimming around in the dark.

“I find it incredibly peaceful,” she says. “Just because it’s hard, doesn’t mean you can’t do it.”

Halligan was in the news recently for a finding a 14,500-year-old tool sharpened on two sides. That tool — or another one like it — had been used to sever the tusk of a mastodon, discovered at the same site. The find was significant because the timeline for mankind’s expansion into Florida remains unclear. The tool would indicate an earlier arrival than many researchers thought the glacial cover of the Rocky Mountains could have allowed for, bolstering the argument that the first Americans travelled by boat down the Pacific Coast.

So how does a woman who was raised in South Dakota and didn’t see the ocean until she was 18 grow up to become an amphibious professional? It’s a compelling question because Halligan is a compelling human, but also because the answer is illustrative of the current state of archeology.

Makeshift rafts become a scientific research platform on the Aucilla River in Florida. Jessi Halligan

Here’s what we know: At the end of the last ice age, so much of the planet’s water was tied up in glaciers that sea levels were about 300 feet lower than they are today. If humans were in North America between 15,000 and 13,000 years ago, they were likely a coastal people, adept at boat making and fishing. It follows that much of the evidence of their existence would now be underwater, submerged by a rising sea. Yet, by Halligan’s estimate there are fewer than 10 archeologists who specialize in looking for sites that were once on dry land, but are now submerged, in North America.

Some in the archeological community are still coming around to the idea that finding good evidence in submerged sites that would prove early North Americans came via the Pacific Coast is even possible. “A lot of scientists say, that’s a great story and it’s plausible, but it’s inherently untestable so it’s not science,” says Halligan, adding that there are more scientists doing this sort of work in Europe, where inundated sites have been found in the strip of land that used to connect the UK to continental Europe. “The coastal route hypothesis is testable, it is able to be verified by scientific findings, you just have to go look.”

Halligan got into the idea of looking for inundated archaeological sites back in her undergraduate years at Harvard, where she trained at a field school on Martha’s Vineyard. The steep cliffs she studied there had once been a hill gently sloping towards the sea. That got her thinking about the interplay between geology and archeology. How do geological processes like erosion and sea level change shape the record left behind by early humans? What is lost, and what might still be out there, waiting to be found?

Jessi Halligan, suited up and ready to go.

The process of excavation underwater is a lot like on land, although it requires a bunch of extra equipment, and costs several times as much money. Divers always go down in pairs, as a safety measure. At the Page-Ladson site in the Aucilla River, where Halligan has done so much of her work, the divers breathe through compressed air hoses connected to the surface, although they also wear full SCUBA gear as a back up. They take off their fins on the bottom, to avoid stirring the sediment. “It’s kind of like the pictures I’ve seen of people doing the moonwalk, because we’ve got all of this gear strapped around our upper bodies but our legs are essentially just wetsuits and booties,” says Halligan. “We kind of bounce around to where we need to be.”

They communicate with each other through hand signals and, as required, written notes in pencil on clipboards with plastic sheeting instead of paper. The divers use a vacuum hose to carry sediment to the surface as they scrape away the layers of earth with trowels. Above the river, the sediment is strained through screens that assistants monitor for anything the divers below may have missed.

Jessi Halligan peels sediment off the floor of the Aucilla River in Florida.

The larger field of underwater archeology also includes shipwreck exploration. “A lot of people start off as people who are SCUBA divers, and they love to SCUBA dive, and they want to figure out a way to combine their interests in history and their interests in underwater life, and they arrived nautical archeology,” says Halligan. She came at it from the other direction — with a curiosity about what archeological artifacts might be found underwater, which necessitated learning how to SCUBA dive.

Halligan's team found a biface stone tool deep within layers of sediment at the Page-Ladson site in Florida.

It was a friend of Halligan’s, who specializes in Medieval European shipwreck archeology, who first spotted the biface stone tool sticking out of the dig site.

“It’s kind of a rule in archeology: The person who is least invested in the project as a whole almost always finds the coolest thing that is found on the project,” Halligan says. She recalls thinking, “He’s a freaking Medieval archaeologist, he’s not going to know that this is something — I was trying to not let my hopes get up, and then I get down there, and it really is a definite tool that is absolutely, for sure, made by people, in this layer that had been dated previously to over 14,000 years old.”

Morgan Smith and Michael Waters examine the discovery. A. Burke

Halligan and grad student Morgan Smith did a happy dance. “We totally had this underwater hug-dance thing that probably went on for 20 seconds.” She says it was a bit like Teletubbies hugging set to the soundtrack of people screaming through regulators.

Halligan is betting that there are other great discoveries out there waiting to be made, and she’s going to go look for them. Good candidate sites can be identified without getting wet by scanning the underwater topography, but after that you still have to locate the funds that facilitate excavation. “It’s not like there’s billionaires just throwing money at us, saying please go look for things,” she says. Still, with more and more cool artifacts being brought up from inundated sites like Page-Ladson, Halligan expects interest in her peculiar field of archeology to grow. And then there’s climate change.

“More of the world is becoming inundated every day,” Halligan says. “I think people are going to get more and more into it.”