TOKYO (AP) — A giant, underground pipe rose more than 10 meters (32 feet) out of a construction site in a busy area of the Japanese city of Osaka, nearly reaching an elevated road overnight, unseen by any witnesses.

The steel pipe’s unexpected growth spurt was reported to police early Wednesday by a pedestrian who saw broken pieces of asphault falling from the cylinder, baffling people passing by and causing traffic congestion.

One office worker who passed by the site told NHK public television that he could not understand how it happened. Another man who works nearby said he first wondered if a new road support might have been built overnight.

The pipe, with a diameter of 3.5 meters (11.5 feet), towered as high as 13 meters (42 feet) at one point, according to the Osaka construction department.

The pipe’s unexpected elevation from the ground occurred at a sewer construction site where workers had been connecting an existing sewer line with a channel designed to hold excess rainwater to prevent flooding.

The pipe was being used as a retaining structure to keep the surrounding soil from collapsing during the operation, officials said. A short time earlier, workers had drained water from the pipe, which may have caused the empty apparatus to float, they said.

By Thursday it had been lowered back to just several feet above the ground after firefighters cut a hole on the side and injected water to push it back into the ground.

City officials said they plan to cut the last 1.6 meters (5.2 feet) of the pipe that remain visible, an operation that would cause a road closure for several more days.

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Video journalist Ayaka McGill contributed to this report.