LONDON — On a finger numbingly cold morning, Lara Maiklem swung open a metal gate tucked behind a pub in southeast London and scrambled down a set of slick stone steps onto the banks of the River Thames.

The river runs through the city west to east, bisecting London as it winds past the new skyscrapers and old docks that line its banks.

But twice a day, the low tide pulls the flowing edges of the Thames back — dropping the river level by 20 feet in some areas — revealing centuries of forgotten London life in the fragments that poke out from the newly exposed land, known as the foreshore.

This is when the mudlarks, like Maiklem, come out.

“What you are looking for are straight lines and perfect circles,” she said, her eyes scanning the surface of the mud for man-made artifacts. “They sort of stand out from the natural shapes.”

Within minutes she had spotted fragments of a 17th-century jug, the half-face of a bearded man visible in the clay.

The name — mudlark — was first given to the Victorian-era poor who scrounged for items in the river to sell, pulling copper scraps, rope and other valuables from the shore. But more recently the label has stuck to London’s hobbyists, history buffs and treasure hunters who scour the river edge searching for objects from the city’s past.

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Mudlarking’s popularity has grown steadily in recent years, driven in part by social media communities in which enthusiasts share their finds and tour groups that offer a trudge through the shards of history’s castoffs.

Dr. Fiona Haughey, a London archaeologist who has worked on the Thames since the 1990s, said that although some mudlarks are looking for valuables, others are looking for a connection with the everyday objects of a bygone Britain.

But it’s the connection with the layers of lives of Londoners before them, revealed by the tides of the river at the heart of the metropolis, that unites the enthusiasts.

For Haughey, a specialist in prehistory, it’s about what an object can tell her about its owner rather than what value it has.

“I love the conundrum of it,” she said.

The Port of London Authority, which owns the Thames waterway along with the Crown Estate (i.e. Queen Elizabeth II), began to regulate exploration along the shore in 2016, requiring anyone searching the banks to have a foreshore permit.

These permits — about 1,500 were issued this year — allow people to explore the terrain, and scrape or dig into the mud up to a depth of 7.5 centimeters (around 3 inches). Mudlarks are advised to report objects that could be of archaeological interest to the Portable Antiquities Scheme, run by the British Museum.

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Stuart Wyatt, the regional finds liaison officer based at the Museum of London who assesses the artifacts, said there was a “huge increase in numbers” of mudlarks in recent years.

“I now have months where I see only new finders,” he said by email.

Britain legally obligates anyone who unearths “treasure” — defined as single finds of gold and silver over 300 years old, and hoards of coins and prehistoric metalwork — to inform the government.

Britain takes this law seriously, as one amateur treasure hunter learned in November. He was given a decadelong jail sentence after failing, along with another man, to report the discovery of a Viking hoard they dug up in western England.

A specialized permit allows deeper digging to a depth of 1.2 meters (3 feet and 11 inches). But those permits are available only to members of the exclusive Society of Thames Mudlarks — an invitation-only group of around 50 members — who have already held a standard permit and reported their findings to the Museum of London for two years.

Some mudlarks bring metal detectors. But most simply recover what the river has naturally revealed, usually a fascinating trinket rather than treasure.

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“I like just to collect what the river decides it’s going to leave on that day,” Maiklem said. “It’s that element of luck.”

But sometimes there are more significant finds, like the first “spintria” found in Britain. Spintriae are Roman bronze tokens, with depictions of sexual acts on one face and a Roman numeral on the other, whose purpose remains uncertain.

And every tide reveals some of the city’s varied story: Roman coins, medieval badges worn by religious pilgrims, an elaborate 17th-century watch.

The Thames, the very reason people began settling in the city over 2,000 years ago, is one of the best preservers of London’s history. The river has been used many ways over the millenniums — as a highway, a source of food and, most important to mudlarks, as a dump.

“The Thames is unpredictable, so its just all mixed up, like a big washing machine,” said Jason Sandy, an architect who mudlarks in his spare time.

In the center of London, where the heart of the Roman city stood, many of the finds are Roman or medieval. Farther west, evidence of prehistoric settlements have been found.

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Where Maiklem was exploring, in Rotherhithe, once an old shipping center in eastern London, finds from the 16th and 17th centuries are the norm.

This particular morning, the sun was just rising and the tide was still on its way out as she clambered over rocks.

Her eyes flicked quickly over the mud, scattered with bits of modern trash, and settled on the barely visible edge of a coin nestled against a wooden post. She plucked it out and wiped away the grime, revealing a George III farthing, the silhouetted face of the monarch and the date, 1777, nearly rubbed smooth.

The coin had been bent into an ‘S’ shape and had a small hole poked through its edge where a chain could be attached, hallmarks that it was likely a love token. At her feet, pieces of clay tobacco pipes from the 16th and 17th century clinked as they washed against rocks, so common as to escape a mudlark’s interest.

But a perfectly round musket ball was worth plucking from the muck. The leather sole of a hand-stitched shoe, preserved by the anaerobic mud, flapped in the breeze, and she tugged at its toes to wrest it from the bank.

Huge wooden beams from the ships broken up here in the 17th and 18th centuries jut out of the mud. The Mayflower is believed to have been broken down here for scrap.

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“There are so many ghosts locked in the foreshore,” Maiklem said. “Also, it’s so fleeting because if it’s not collected from the surface, it’s going to be washed away or broken down.”

Maiklem, who has spent more than 15 years exploring the river’s banks, takes only the most unusual items home with her. She sees her discoveries as part of a shared history and uses social media to reveal her finds. She has more than 100,000 people following her.

While Maiklem recently moved out of the city, she still makes the journey to the Thames weekly, driven by the thrill of discovery.

From here, the hustle of London seems a world away, with gulls cruising between the barges and the old warehouses turned luxury apartments that stand on the north side of the river, a sign of the ever-changing city.

The Shard — London’s tallest and one of its most recognizable skyscrapers — juts in the distance, reflecting the morning light from its thousands of glass windows.

“It’s a way of just escaping from all of this controlled chaos,” Maiklem said, gesturing to the skyline. “This is what London is about for me.”