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 might 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. Tagliabue apparently made a thermometer for Cooper Union, “one of the largest ever manufactured.”

At the moment, there appears to be no danger to the children or anyone else, as long as the asphalt that covers the lot remains in place, acting as a cap.

The danger could occur if the cap is removed. And in construction-happy New York, that is always a possibility.

The Howard Hughes Corp., which controls a significant portion of the South Street Seaport under a lease with the city and has been updating the area, recently purchased the parking lot, adding to its Seaport empire.

Although it has no approved development plans for the site, Howard Hughes is proposing to remediate it, which typically is done right before building starts.

South Street Seaport, a popular tourist spot since the 1980s, is a designated historic district. A vestige of New York’s early days as the country’s leading port, this roughly 10-block hub on the East River contains a unique collection of early-19th-century brick buildings. They stand just four or five stories high on cobblestone streets, some of which now house shops and restaurants. Nearby are several residential towers.

From the river to lower Broadway, extending slightly past the district, about 14,500 people lived in the area in 2016, according to the American Community Survey. And the population is growing.

Many residents are already resistant to the neighborhood’s losing its historic character; that toxic mercury could be underneath 250 Water St. and released around children has come to underscore the tension between residents and the real estate developer over the future of the Seaport.

“We’re opposed to overcommercialization, overdevelopment and toxic contamination,” said Elaine Kennedy, a retired physical therapist who has lived across the street from the parking lot for 43 years and is a member of the nonprofit Save Our Seaport.

Simeon Bankoff, executive director of the Historic Districts Council, a preservation group, said the Seaport was no place for “an enormous anywheresville tower that will drain the life out of it like some weed that overshadows all the small plants.”

But some development experts believe that it’s time to update the Seaport. “We can’t as a city allow a critical site like this one to remain undeveloped when there’s so much need for housing, for commercial space,” said Seth Pinsky, executive vice president of the developer RXR and a former president of the city’s Economic Development Corp.

Howard Hughes is “doing what a developer does in terms of trying to build on the parking lot,” said Lynne Sagalyn, a real estate professor emerita at the Columbia Business School and author of “Power at Ground Zero: Politics, Money, and the Rebuilding of Lower Manhattan.” Speaking more broadly of the neighborhood, she added, “That area has languished too long.”

The Seaport issue comes at a time when high-rise developments are cropping up all over the city, as developers maneuver around zoning rules while promising to provide much needed affordable housing.

On Manhattan’s Lower East Side, for example, Extell Development replaced a supermarket with an 80-story condominium (and provided an affordable housing development nearby). Proposals for three more supertalls in the area have prompted lawsuits.

“The buildings are too tall for this site,” said the Manhattan borough president, Gale Brewer, who wants the development proposals for the Lower East Side to go through the city’s Uniform Land Use Review Procedure rather than gain approval, as a “minor modification,” from the City Planning Commission. “It destroys the character of the neighborhood,” she said, “not to mention the view of the Manhattan Bridge.”

But while the Seaport has concerns in common with other New York neighborhoods when it comes to aggressive development, it is an undeniably special place. And many want it to stay that way.

“People go down to the Seaport because it speaks to our collective memory, our collective thoughts about what is New York City,” Bankoff said.

The Seaport is both a city and federal landmark. The National Trust for Historic Preservation put the district on its list of America’s 11 Most Endangered Historic Places in 2015.

So when Howard Hughes, a publicly traded company that specializes in master-planned communities, proposed a 50-story hotel and condominium on the waterfront several years ago, community members, elected officials and preservationists united to quash the plan.

In the aftermath, the parties formed the Seaport Working Group, an advisory board that Howard Hughes joined. Everyone agreed to guidelines for the district, which included that development “should not adversely impact neighborhood scale and character.”

Community members say any development for the parking lot site that exceeds the 120-foot height limit in the area would adversely impact the neighborhood’s character, and they want a development plan to be in place before the remediation process begins.

The site at “250 Water St. is becoming a hostage situation,” said Grace Lee, a parent and local activist.

Saul Scherl, president of the New York tri-state region for Howard Hughes, thinks some NIMBY-ism is at play. “I believe they have an alternative agenda to maintain it forever as a parking lot,” he said.

Sagalyn, the professor, said that “keeping it as a parking lot is untenable,” adding that Howard Hughes is “trying to do something to reactivate the site and reintroduce New Yorkers to the Seaport.”

But residents feel their wariness is warranted. Many were irked when the developer started calling the South Street Seaport the Seaport District, a name intended to encompass the new restaurants and upscale shops there. When the company revealed plans for a parklike roof atop Pier 17 and instead delivered a thumping concert venue, many saw the move as a betrayal.

“They snowed us,” said Stacey Shub, a resident and parent. “We were promised a beautiful roof with grass, a play area for the kids.”

Scherl acknowledged the criticism of Pier 17. “The reality of it is,” he said, “it’s also a for-profit.” He added that the rooftop has offered ice skating and movie nights in the past, but that the company is still trying to find the right balance between meeting business goals and serving the community. “The concerts bring a sense of energy, a vibe,” Scherl said. “That’s something we need to bring in for the restaurants and retail.”

Last year Pier 17 sold out 18 of 23 shows and welcomed more than 60,000 guests, according to Howard Hughes. Billie Eilish, the young pop singer, will perform there this month in a sold-out show. Pollstar, a publication that covers the concert industry, voted it the “best new concert venue” in the country.

Scherl said he has sought to address noise complaints and has supported local nonprofits. He has also tried to throw a lifeline to the perennially struggling South Street Seaport Museum, which, like the rest of the area, was deluged during Hurricane Sandy.

But the parking lot atop the thermometer factory has become what Kennedy, the retired physical therapist, described as a Pandora’s box for the developer, serving to galvanize frustrated residents, who are demanding that Howard Hughes become more transparent about its plans.

In November, Howard Hughes applied to the state’s brownfield cleanup program, which encourages private-sector remediation in exchange for a release from liability if contamination is later discovered on or seeping from a remediated site. It also offers tax benefits in certain instances, such as the inclusion of affordable housing.

“We have a fiduciary obligation to clean up the site,” Scherl said.

But before any remediation takes place, residents want to know what the developer intends to build at 250 Water St. The neighborhood’s Community Board 1 recently passed a resolution — what amounts to an official recommendation — calling for no activity on the 1-acre parking lot until an approved land-use plan is in place.

Indeed, the community has been asking Howard Hughes about its plans across the entire Seaport, where the developer leases Pier 17, the adjacent Tin Building site and portions of buildings on Beekman, John, South, Fulton, Front and Water streets.

Last week, Howard Hughes announced that it was moving in that direction: It has hired architecture firm Skidmore, Owings & Merrill to lead a master plan process that would address the Seaport as a whole and the specific sites Howard Hughes controls or has an interest in, including the parking lot.

“The hope is to look at the site comprehensively and come up with a series of programs, architectural language and building masses that build a synergy between all pieces,” said Chris Cooper, an architect and a Skidmore design partner who will help lead the planning process.

So far, the course of action involves holding community workshops and creating and updating a website, Cooper said.

As for the parking lot, based on statements from Howard Hughes — and on the $180 million the company paid for the property — the developer seems interested in erecting a building or buildings on it that could very well exceed the height limit for the site under current zoning, which allows for only 12 stories.

A plan for the parking lot is “exactly what we and Skidmore Owings & Merrill will be figuring out over the next months in close consultation with community stakeholders,” Scherl said.

Howard Hughes has expressed a desire to transfer air rights from other properties to the parking lot site. It has said it could have more than 600,000 square feet available, but the city disputes that amount. Also the lot is not a “receiving” site for air rights. For the transfer to take place, the city would have to make an exception.

But some city officials said they were not interested in more exceptions for the Seaport. “Everything everyone does is tall,” Brewer said. “I want development here to stay at 120 feet.”

Others feel differently. “When you look at a surface parking lot in a prime location in lower Manhattan,” said Pinsky of RXR, “and you think about the tax revenues that are not being generated and the jobs that are not being created, the housing that is not being supported, it’s hard to believe there is not a way to develop the lot that is sensitive to the issues the opponents raise but that puts the property back into more productive use.”

Regarding the mercury issue, Brewer noted that the parking lot is in the flood zone, echoing community members who worry that a storm surge during remediation could wash up contaminants into the neighborhood. “It needs very intensive oversight,” she said.

Brewer and other elected officials recently held a meeting with the state Department of Environmental Conservation, which administers the brownfield program and monitors cleanups of sites in the program, so that they could express their concerns.

Elemental mercury can vaporize when exposed to air, and if vapors are inhaled they can cause brain damage, according to Anthony Carpi, an environmental toxicologist at the John Jay College of Criminal Justice.

Carpi said that exposure to mercury vapors is a particular concern for children, whose nervous systems are still developing. And because the respiratory rates of children are higher than those of adults, he added, they “are likely to take in more of it.”

In the case of the parking lot, air monitoring would have to take into account the fact there are so many students nearby (in addition to Peck Slip, there is also the Blue School and several day care centers).

So far, there has been no comprehensive study of the parking lot area, so no one knows whether “the site is slightly contaminated or grotesquely contaminated or something in between,” Carpi said.

Of the 146 New York City sites approved for the brownfield cleanup program since 2003, none have involved elemental mercury contamination. When such contamination was found in a backyard cottage in Ridgewood, Queens, which had been used by the Weixler Brothers Thermometer Co. to make thermometers for submarines in the first half of the 20th century, the conservation department referred the site to the federal Environmental Protection Agency for remediation. The cottage was demolished as part of the cleanup. The mercury was eliminated.

The conflict over the parking lot has united young parents with longtime locals, such as the members of Save Our Seaport. New York City Comptroller Scott Stringer and Assemblywoman Yuh-Line Niou recently joined a community protest of Howard Hughes’ handling of the parking lot.

Residents said they were hiring an environmental consultant to independently monitor work on the site. One parent, Megan Malvern, has borrowed an air-monitoring meter so she can take readings around the site.

Although the future of 250 Water St. is uncertain, for many residents, one thing is for sure. “We’re getting overdeveloped,” said Kennedy, the retired physical therapist. “And we’re drawing the line at the Seaport.