There was no masked robber with a threatening note for the bank teller, no bag bulging with cash, no getaway car.

Instead, whoever appeared to have plotted a heist at a Chase bank in Pembroke Pines, Fla., chose a rather unglamorous, subterranean approach. The plan involved a narrow, 150-foot-long tunnel dug into the dirt.

Officials discovered the tunnel on Tuesday night after public works employees in Pembroke Pines, about 20 miles north of Miami, went to deal with a sinkhole in the road. Curiously, the workers found a power cord inside it. Realizing it was a tunnel, they eventually called the local police, who notified the F.B.I. on Wednesday morning.

F.B.I. investigators found that the tunnel had a diameter of about three feet and opened up into a wooded area, Special Agent Michael D. Leverock said in an interview on Wednesday evening. It was so narrow that a person would have to lie on his or her stomach to navigate it, Agent Leverock said.