The boy was rescued after being swept away and spending more than 12 hours in the city's toxic and maze-like underground sewer system.
Jesse Hernandez had been playing with other children on wooden planks over an access portal to the sewer system during a family outing on Sunday at a Los Angeles park.
When a plank broke, Jesse fell about 25 feet and landed in fast-moving sewage, said Brian Humphrey, a spokesman for the Los Angeles Fire Department.
The other children immediately notified adults, who called 911, initiating a frantic and exhaustive 12-hour search of labyrinthine underground pipes using cameras propped on flotation devices.
Rescuers finally found Jesse after seeing images of handprints on a sewage pipe. A sanitation crew rushed to the area and opened a manhole.
"The first thing they heard was 'Help!'," said Adel Hagekhalil, assistant general manager of the sanitation department.
The crew lowered down a hose to Jesse, who was about 11 feet deep in the pipe.
"He caught on to the hose and was reeled back up," Mr Hagekhalil said.
Rescuers gave him immediate medical attention, including hosing him down to get rid of the sewage and cleaning out his eyes and nose with sterile saline, Mr Humphrey said.
Jesse immediately asked for a mobile phone to call his family. A worker handed him a phone, and he called his mother.
The boy later told KNBC-TV that he was tired but otherwise fine.
"I was praying to God to help me and to not die," Jesse said. "It was all quiet. You could just hear the water running through and you couldn't see anything. It was dark."
Video released by the city later in the day showed long dark marks on the sides of the dank sewage pipe, which was about four feet in diameter, where Jesse's fingers would have slid.
After an accident like Jesse's, rescuers said the likelihood of someone being found safe diminishes by the hour. Mr Hagekhalil said rescuers were thrilled at the outcome.
"They never gave up hope," he said. "They wanted to bring Jesse back to his family."
About 2,400 feet of pipe had been inspected when rescuers found Jesse less than a mile from where he disappeared. He was taken to a hospital for a full evaluation and to be reunited with his family.
In addition to the massive rescue effort involving more than 100 people, Mr Humphrey credited Jesse for his survival.
Not only did he survive getting swept through sewage moving at 15mph, he managed to find a pocket of breathable air and hang on until he was found, authorities said.
"Any subterranean location, particularly one that involves waste, can produce toxic gases - methane, hydrogen sulfide - so breathable air is a key element," Mr Humphrey said.
"The odds of someone falling into such a pipe and surviving are slim. The odds were not in his favour, and many would call it miraculous."
He called Jesse "a resolute young man".
"He has tremendous inner strength," Mr Humphrey said. "It's obvious he's not a quitter."
Los Angeles mayor Eric Garcetti was among the many people closely watching the rescue efforts for Jesse.
In a string of tweets, Mr Garcetti praised rescuers and thanked the teenager's family for their "patience and optimism during this harrowing experience".
"We're keeping Jesse's health and recovery in our thoughts today," he said.
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