Recent sinkholes on Interstate 80 in North Jersey have led to the closure of sections of the highway. The sinkhole problem may be due to old mine shafts beneath the area, a legacy of the region's history as a mining hotspot. Officials are investigating the cause as local geologists point to potential collapses triggered by vibrations from traffic.
NJDOT photo
In early February 2025, a sinkhole 11 ft. in diameter opened up along the eastbound lanes just west of the interstate's New Jersey Highway 15 overpass near Wharton.
Once called the Bergen-Passaic-Delaware Expressway, Interstate 80 in northern New Jersey was a $387 million mash-up of projects built more than 50 years ago.
Today, one is not working out well.
In early February 2025, a sinkhole 11 ft. in diameter opened up along the eastbound lanes just west of the interstate's New Jersey Highway 15 overpass near Wharton. After the collapse, New Jersey Department of Transportation (NJDOT) officials said they found 90 spots to check for possible instability or voids in the area.
Weeks later, an even larger sinkhole opened there, leading to the full closure of the highway, NorthJersey.com reported March 24, 2025.
State officials still do not know, or will not say, what exactly has caused the sinkholes. The extent of the problem is unclear, as is a lasting solution.
The I-80 stretch from Denville to Roxbury was built generally to parallel U.S. Highway 46 through North Jersey as part of the Interstate Highway System, conceived in the mid-1950s. The idea was to use the federal program and its dollars to carry U.S. 46's traffic through Dover, Wharton and other "bottleneck areas," the Paterson Evening News reported in August 1959.
The section appropriately started where U.S. 46 narrows from four lanes to two heading west.
S.J. Groves and Sons of Woodbridge built the stretch from Mount Hope Road to N.J. 15 for $5.5 million in time for the highway's opening in the fall of 1959, according to a report from the Paterson newspaper at the time.
One year later, the 3.7-mi. section of I-80, where the 2025's sinkholes have appeared, running from N.J. 15 to Howard Boulevard, was opened to traffic. The state's Highway Department awarded that $6.7 million contract to George M. Brewster & Son of Bogota, then Bergen County's largest road contractor and a key builder of the New Jersey Turnpike.
What is Under Interstate 80's Lanes?
It appears the sinkhole problem, however, may have far more to do with how and where the highway was built than by whom, noted NorthJersey.com. Decades before the construction, Wharton was an iron mining town and its mine district follows a linear trajectory cutting northeast from Mine Hill to Mount Hope.
Dozens of mines were dug throughout the area. Some were short-lived and never amounted to much. Others cut more than 1,000 ft. into the fault-laden ground in search of iron-bearing magnetite amid belts of dense gneiss.
New Jersey's iron mines may be long closed, but development in the northern part of the state continues to run up against the effects of past mining, noted the 2024 New Jersey State Hazard Mitigation Plan.
Subsidence remains a concern, as some abandoned mine locations and their associated shafts, openings and entrances are mapped inaccurately, the state said. Others have been improperly filled and capped, a situation potentially made worse by the considerable seismic activity in the Tewksbury area last year.
Roads and buildings also have been constructed on or near these sites, adding to the risk, the 2024 plan noted.
The New Jersey Department of Environmental Protection's (DEP's) GPS-based mapping system pinpoints the location of the Mount Pleasant Mine as south of I-80 and west of N.J. 15, or directly under the Avalon Wharton housing complex, which lies just to the south of the sinkhole zone.
A 101-year-old map, complete with underground mapping from the local Replogle Steel Co., and old U.S. Geological Survey records placed the mine's primary shaft in that general area, with under workings to the southwest and northeast that are hundreds of feet underneath the highway down to below sea level.
NorthJersey.com learned from 1958 survey reports conducted before the freeway was built that most of the ore was extracted northeast of the shaft before it closed in 1896, and the shaft itself had "caved" by the time I-80 was constructed through Wharton.
DEP officials have marked the modern geographic information system maps "authoritative," but metadata warnings have said that the map last updated in December 2023 is not verified by DEP officials and is it not authorized or endorsed by the state, the online news source reported.
New Jersey Was Once Key Mining State
Beyond the accuracy of even modern maps, none are thought to include all the shaft openings, test pits, entrances and even entire mines that were constructed during New Jersey's history as a mining hotspot prior to westward expansion.
Mines are scattered throughout North Jersey's Highlands, but records — especially those predating the mid-19th century — are often unreliable, said Alexander Gates, a Rutgers-Newark geologist.
Gates said he has not studied Interstate 80's sinkholes specifically but believes the region's mining history is a likely factor.
Towns and oversight officials may have required mapping, but early shafts were lit by candles, and compasses were mostly useless due to magnetite in the rock. As a result, mine tunnels may not run in the expected directions, and older shafts could be closer to the surface than previously thought, Gates said.
"Miners followed the veins," he added.
Almost 600 Abandoned Mines Are in North Jersey
Northern New Jersey's rock is particularly hard, but if a mine shaft is near the surface, constant vibration from traffic could trigger a collapse, according to Gates.
Officials at the DEP would not comment on the highway sinkholes' possible connection to the area's mines and directed questions to CTDOT, which also declined to answer inquiries about the sinkholes' suspected cause, whether it be from a mine collapse, construction flaws, or a combination of those and other factors.
In the past, officials with the state Geological and Water Survey, a division of the Connecticut DEP, have said the area's deep underground mine tunnels and voids are generally safe, apart from shaft openings and entrances.
However, there have been collapses before, NorthJersey.com noted.
In the 2000s, underground voids associated with the White Meadow Mine in nearby Rockaway Township caused cave-ins under a public roadway and a home, requiring the use of a boulder-and-concrete matrix to cap shafts, and low-mobility grouting to fill voids, according to local records. In that case, microgravity geophysical surveying and test drilling helped look for voids.
In the past 50 years, Rockaway Township and other towns, such as North Arlington and Mine Hill, have spent hundreds of thousands of dollars on remediation, state records show.
The problem is not a small one, as nearly 600 abandoned mines — including copper, graphite, iron, lead, mica, manganese, sulfide, uranium and zinc — have been identified in North Jersey. Most are concentrated in Morris, Sussex, Warren, Hunterdo and northern Passaic counties.
The existence of the mine infrastructure in the Wharton area also caused other complications for George M. Brewster & Son Inc. decades ago.
Before I-80, there were well-established transportation arteries to contend with. The company had to construct the highway over North Main Street in Wharton, the old Mount Hope Mineral Railroad and U.S. 15.
In addition, retaining walls were built to bolster short-decked spans over the obstacles and hold back the massive amounts of fill used to build up the freeway between North Main Street and Green Pond Brook, a tributary of the Rockaway River, on the other side of U.S. 15 to achieve the elevations needed to make the crossings.
Timothy Bechtel, a senior geosciences professor at Franklin & Marshall College, told NorthJersey.com on March 20, 2025 that water is a likely factor in North Jersey's sinkhole problem, pointing to precipitation or leaking underground pipes as common causes. Sinkholes like the ones that opened along the interstate are "usually related" to these conditions, he said.
Satellite images of the area from 25 years ago show a grass-filled median where the sinkholes have formed. Today, the median between the old railway and the brook is made up of rocks and shrubbery, a clear sign of erosion.
Gates said that if runoff were the cause, the ground would likely bow before caving in. The way these sinkholes have formed suggests that abandoned mine shafts are to blame, he added.
That could be an issue to the west of Wharton on I-80, with other known mining zones close to the highway in Roxbury, Mount Olive, Byram and Allamuchy. The nearby Huff mine also is located right under the interchange of North Main Street and I-80 in Wharton on the DEP's geographic information system map.
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