The man was reported missing around 6:45 p.m. on Sunday, Sept. 29, and following the call for help Concord, Lincoln and Bedford Police, along with state police helicopters and dive teams began searching the pond made famous by Henry David Thoreau.
On Monday, the search focused on the "left" side of the pond, as the man was thought to be swimming on toward or from the Lincoln shoreline after departing form the main beach.
Walden Pond, a state reservation, was shut down on Monday, as the Department of Conservation and Recreation, did not want swimmers and visitors to interfere with the ongoing search.
Around noontime, the wakes left by search vessels and hum of a generator inside the State Police trailer were the only interruptions in an otherwise serene autumnal setting.
Beyond the boats and divers on the water, the Mass State Police Air Wing circled overhead and mounted units searched the woods around the pond.
Situated in a state reservation, Walden Pond is a glacial kettle pond more than 100-feet deep at some points. The pond has also been designated as a National Historical Landmark, a title owed at least in part to the time Henry David Thoreau spend there in the mid-1800s and its prominence in his work.
Click through the photos above from some scenes from the scene on Monday. And stay tuned: Concord Patch will bring you more information on this story as it becomes available.