They found no trace of a mythical sea monster, no sign of mobsters in concrete shoes or long-lost treasure chests.

But scuba divers who spent a year cleaning up Lake Tahoe’s entire 72-mile shoreline have come away with what they hope will prove much more valuable: tons and tons of trash.

In addition to removing 25,000 pounds of underwater litter since last May, divers and volunteers have been meticulously sorting and logging the types and GPS locations of the waste.

The dozens of dives that concluded this week were part of a first-of-its-kind effort to learn more about the source of, and potential harm caused by, plastics and other pollutants in the storied alpine lake on the California-Nevada line.

It's also taken organizers on a journey through the history, folklore and development of the lake atop the Sierra Nevada that holds enough water to cover all of California with 14 inches of water.

The Washoe Tribe fished the turquoise-blue Tahoe for centuries before the mid-1800s, when the state sanctioned the genocide and removal of Native peoples to serve the interests of railroads and timber barons. The Tahoe area became known for its decadence, a playground for the rich and famous.

Tahoe’s first casino was built in 1902 by Elias J. “Lucky” Baldwin, who owned a big chunk of east Los Angeles and built the prominent Santa Anita horse track in 1907. Massive lakefront estates followed for decades, including one used for the filming of “Godfather II."

Cleanup organizers say one of the things locals ask most is whether they’ve found any gangsters’ remains near the north shore. That’s where Frank Sinatra lost his gaming license for allegedly fraternizing with organized crime bosses at his Cal-Neva hotel-casino in the 1960s.

The recovered debris mostly has consisted of things like bottles, tires, fishing gear and sunglasses.

But Colin West, founder of Clean Up The Lake, the nonprofit environmental group that launched the project, said there have been some surprises.

Divers think they spotted shipwreck planks near Dead Man’s Point, where tales tell of a Loch Ness Monster-like creature — later dubbed "Tahoe Tessie" — living beneath Cave Rock.

They've also turned up a few “No Littering” signs, engine blocks, lampposts, a diamond ring and “those funny, fake plastic owls that sit on boats to scare off birds,” West said.

“It’s shocking to see how much trash has accumulated under what appears to be such a pristine lake,” said Matt Levitt, founder and CEO of Tahoe Blue Vodka, which has contributed $100,000 to the cleanup.

His business is among many — including hotels, casinos and ski resorts — dependent on the 15 million-plus people who visit annually to soak up the view Mark Twain described in "Roughing It" in 1872 as "the fairest picture the whole earth affords."

“It is our economic engine,” Levitt said.

And while most contributors and volunteers were motivated primarily by helping to beautify the lake, scientists are excited by what happens once the litter is piled ashore.

Shoreline cleanups have occurred across the nation for years, from Arizona to the Great Lakes, Pennsylvania and Florida. But that litter goes into recycling bins and garbage bags for disposal.

Each piece from 189 separate Tahoe dives to depths of 25 feet was charted by GPS and meticulously divided into categories including plastic, metal and cloth.

Plastics are key because international research increasingly shows that some types can break down into smaller pieces known as microplastics.

Scientists are still studying the extent of, and human harm from, the tiny bits. But the National Academy of Sciences said in December that the U.S. — the world’s top plastics-waste producer — should reduce plastics production because so much winds up in oceans and waterways.

Zoe Harrold, a biochemist, led scientists at the Desert Research Institute in Reno, which first documented microplastics in Tahoe in 2019. She was lead author of Clean Up The Lake’s 2021 report on a 6-mile pilot project.

“If left in place, the ongoing degradation of submerged litter, particularly plastic and rubber, will continue to slowly release microplastics and leachates into Lake Tahoe’s azure waters,” Harrold wrote.

The cleanup comes a half-century after scientists started measuring Tahoe’s waning clarity as the basin began to experience explosive growth.

Most people credit, or blame, completion of the interstate system for the 1960 Winter Olympics near Tahoe City. The first-ever televised Olympics introduced the world to the lake surrounded by snow-covered peaks.

From 1960-'80, Tahoe’s population grew from 10,000 to 50,000 — 90,000 in the summer, said the U.S. Geological Survey. Peak days now approach 300,000.

“The majority of what we’re pulling out is a result of basically just the human impact of recreating, living and building a community here in the Lake Tahoe region,” said West, of Clean Up The Lake.

His group is planning dives this year at other Sierra lakes, including June Lake east of Yosemite National Park, and will expand future Tahoe searches to deeper depths.

The nonprofit Tahoe Fund, which also helped raise $100,000 for the cleanup effort, is commissioning artists to create a sculpture made from Tahoe's trash at an events center being built in Stateline, on the lake's south shore.

"Our hope is that it will inspire greater environmental stewardship and remind those who love Lake Tahoe that it's up to all of us to take care of it,” Tahoe Fund CEO Amy Berry said.