The woman was driving through a commercial corridor with her window down Tuesday when a scream rang out from a storm drain.
She pulled onto the side of the road, got out of the car and looked down the eight-foot-deep sewer.
A woman was sitting there, naked and pleading for help.
The driver whipped out her phone to call 911, according to a police report.
"Hi, yes, there's somebody stuck in the sewer over here on Atlantic and 11th Avenue!" she yelled to Delray Beach, Fla., police.
"I'm sorry, somebody's stuck in the sewer?" the dispatcher asked.
The driver turned to the woman in the drain while the dispatcher kept talking, according to audio of the call: "Yeah, the cops are on their way, okay?!"
The bizarre discovery led to an eventful rescue operation that culminated in onlookers cheering as first responders pulled the 43-year-old woman to safety with a ladder and a harness. The woman was brought to a hospital to recover while police in Delray, about 50 miles north of Miami, try to figure out what happened to her.
The Washington Post is not naming her because police records suggest that she has a history of mental illness and drug abuse.
The woman's boyfriend had reported her missing to Palm Beach County sheriff's officers nearly three weeks earlier, when he left for his job at a car dealership while she was sleeping and returned to find her gone. Her cellphone and purse were still on the kitchen counter and chair, according to a missing-person report that he filed.
First responders who found the woman in the sewer Tuesday removed the grate covering the drain and lowered the ladder into the hole, Dani Moschella, a spokeswoman for Delray Beach Fire Rescue, told the television station WPTV. When the rescuers realized that the woman was too weak to climb up, they used the harness to pull her to ground level.
Although the woman was last seen wearing a yellow sweatshirt and gray pants, she was found naked, dirty and covered in leaves. Her knees were scraped, and she had superficial injuries, Moschella told reporters. Rescuers held white sheets around the woman to protect her privacy.
The small size of the storm drain's opening makes it unlikely that the woman entered at that spot, Moschella told WPTV. Two pipes connect to the location.
The woman told officials a strange story about how she had gotten there, according to the police report: She was swimming in a canal near her boyfriend's house on March 3 when she noticed a door leading to a tunnel. Curious where it led, the woman said, she entered and kept following a series of tunnels until she realized that she was lost.
She wandered the sewer system for about three weeks, she told police, until she saw light above her and people walking around. She said she decided to sit in that spot.
Police are trying to determine whether her story could be true. The woman was found three miles from where she claimed to have entered the tunnel, said Ted White, a spokesman for Delray police. She told officers that she survived on an unopened bottle of ginger ale that she had found.
"We don't feel that there was any crime committed," White said. "But the biggest question is, is her story credible? Was she actually down there the whole time?"
Health officials said the woman was most likely to have been in the sewer system for two or three days, White said.
As hard as police may try to fill in the blanks in the rescued woman's story, Moschella told WPTV, they may never get the full picture.
"Part of this," she said, "might just remain a mystery."
Track Latest News Live on NDTV.com and get news updates from India and around the world