A Port Authority Police Department banner is posted on a barbed wire fence outside Ground Zero and One World Trade Center on March 21, 2014. View Full Caption Andrew Burton/Getty Images

MANHATTAN — An underground leak has been discovered within the World Trade Center complex — and officials fear the seepage may be coming from the slurry wall that separates the newly rebuilt Ground Zero site from the Hudson River, DNAinfo New York has learned.

Workers began to hear the sound of rushing water behind the walls of lower concourses of the complex within the last two weeks, according to sources.

The discovery prompted the Port Authority to quietly call in engineering and construction experts to try to identify its cause, sources said.

Crews were also tasked with dismantling sections of walls and other previous construction along lower subterranean concourses to try to get to the running water and trace its origin. The work is expected to be extremely costly, sources say.

Sources say officials are concerned that the leak may be coming from a stretch of the 3,200-foot-long slurry wall that is hidden by other walls that house unopened commercial offices, retail shopping stores and underground warehouse space that are expected to be operational by next summer.

They fear that the slurry wall may not have been properly insulated, allowing water to seep through it, sources said.

The Port Authority spent tens of millions of dollars since 9/11 repairing the slurry wall after the Twin Towers collapsed.

When it was built in the 1960s, the slurry wall was hailed as an engineering feat withholding the massive pressure of the Hudson River and giving construction crews the ability to open a massive 16-acre hole from which the original World Trade Center rose.

The slurry wall is 4 feet thick and roughly 100 feet deep. Although a stretch along Liberty Street shifted more than 10 inches on 9/11, it managed to keep the Hudson River from breaking through and drowning the smoldering Ground Zero site with water.

The wall’s emotional significance was immortalized when a portion was left exposed inside the National September 11 Memorial Museum.

Asked about the running water and potential slurry wall issue, a Port Authority spokeswoman said engineers had no reports of running water or any known potential issues involving the site’s slurry wall.

She even emailed photos of sections of the slurry wall that are visible along the PATH train tunnels to demonstrate that they are dry and intact.

However, the sections of slurry wall that are of concern to the officials are hidden from view by the subterranean concourses, sources say.

Depending on the severity of the problem, sources say, it could further delay the opening of the remaining concourse commercial space.