On Avenue of the Americas, there is a block where the first cellphone call was completed in 1973; on West 125th Street, where the old Blumstein’s department store stood, nothing marks the place where the Rev. Dr. Martin Luther King Jr. was stabbed in 1958.

Then there is the spot on Fifth Avenue where Winston Churchill, crossing against the light, was struck by a car in 1931 and nearly killed....

Andrew Carroll, 39, an amateur historian, is embarking this week on a 50-state journey to uncover, memorialize and preserve these and other sites where history happened serendipitously, and which, for one reason or another, have been relegated to anonymity.

“It’s sort of a reverse scavenger hunt,” he said. “Trying to find things that aren’t there.”