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 Ms. 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.”