Just after Christmas last year, the eastbound shoulder along a New Jersey stretch of Interstate 80 parted like an asphalt curtain. It left a hole 40 feet wide, 40 feet long and deep enough to imagine the worst.

Crews from the state’s Department of Transportation worked around the clock to repair the ominous breach with wire mesh, stones, concrete and several layers of asphalt. Within four days, I-80 traffic resumed as if nothing had happened. As if the New Jersey earth had not suddenly opened up in the town of Wharton.

But then it happened again, in early February, just two dozen yards from the first breach. A just-noticed depression in the roadway suddenly collapsed, leaving a sinkhole large enough to swallow an S.U.V.

And then again, in mid-March. A 25-foot-deep sinkhole yawned open along the median in the eastbound work zone, followed by detection of a significant void under the westbound lane. A slice of interstate was closed in both directions, while state transportation officials grappled with an infrastructural problem whose cause they knew:

The past.

As I-80 cuts through the New Jersey highlands, enduring the east-west movement of 110,000 vehicles every day, it passes over at least two of the many abandoned iron mines that honeycomb the area. These are the remnants of a thriving ore-and-mineral industry that began before the American Revolution and lasted into the 1970s.

The names on the exit signs — Hibernia, Rockaway, Mount Hope, Dover, Wharton, Mine Hill — are of communities once defined by their dependence on iron. The people who settled here — the English and Welsh, the Irish and Eastern European, and, later, the Puerto Rican — worked away at the bedrock below, following its veins ever deeper into the underworld until the market dried up or the costs became prohibitive.