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In an area as old as the South Street Seaport, a part of Lower Manhattan that dates to the 17th century, historic remnants are bound to turn up from time to time. A wooden ship was unearthed on Water Street in 1981. A stoneware jug was discovered on Pearl Street where a boardinghouse stood in the early 1800s.

And on Peck Slip, where assorted small-scale buildings were lined up in the 18th and 19th centuries, clay smoking pipes and pieces of an old chamber pot were found in 2013 and 2014, when a post office was converted into the Peck Slip School. Students there study the artifacts as part of a fifth-grade archaeology unit.

But these days, their parents and other Seaport locals are more concerned about what may lie under the parking lot across the street from the school: elemental mercury, the toxic remains of a thermometer factory active in the 1800s.

Nineteenth-century maps and records show that Giuseppe Tagliabue, an Italian immigrant, had a five-story factory here in which mouth-blown glass tubes were filled with liquid mercury. Mr. Tagliabue apparently made a thermometer for Cooper Union, “one of the largest ever manufactured.”