A sinkhole in the city of Yashio, Saitama Prefecture, caused by a ruptured underground drainage pipe underscores the growing risk posed by Japan’s aging water infrastructure, experts have warned.

The ground underneath an intersection in Yashio caved in on Tuesday morning, trapping a truck and its driver inside. The sinkhole, initially 5 meters wide and 10 meters deep, expanded to 40 meters in width and 15 meters in depth following several subsequent collapses, according to fire department officials quoted by Jiji Press. Work to rescue the driver is ongoing.

Damaged wastewater pipes caused 2,600 sinkholes in the year from April 2022, the land ministry has reported, though most of them were less than 50 centimeters deep.

Yashio’s sinkhole is much larger than most because of the size of the ruptured pipe — it has a diameter of 4.75 meters — and its placement 10 meters underground, causing large volumes of mud to sink into it, said Shinya Inazumi, a professor of civil engineering at the Shibaura Institute of Technology in Tokyo.

The hole likely expanded because the ground where the pipe was buried was soft, which made the earth and sand underground move quickly toward the hole, he added.

“Such big drainage pipes are all around the country, and most cities in Japan are built on soft ground,” Inazumi said. “In that sense, an accident like this could happen anywhere.”

The exact cause of the damage to the pipe in Yashio remains unclear, but hydrogen sulfide may have corroded the concrete pipe, which was built in 1983, according to the Saitama Prefectural Government. The colorless gas — formed from human waste and detergents contained in wastewater — is known to mix with bacteria to become sulfuric acid and corrode concrete.

Saitama Prefecture checks sewage pipes every five years, and during the inspection conducted in 2021, the pipe underneath the Yashio intersection was given a B out of a ranking from A to C, with A indicating the most serious level of corrosion, and therefore judged as not requiring immediate repairs.

Inazumi said these routine checks may have been insufficient, adding that more frequent and detailed checks are necessary.

“The public should wake up to the fact that this is not a one-off accident,” Inazumi said. “We should recognize sinkholes as a big disaster risk.”

Hiroki Tanikawa, a professor and urban sustainability expert at Nagoya University, agrees that aging sewer systems elsewhere in the nation face similar issues, pointing to the risks of iron reinforcing bars inside the concrete being corroded by water seeping through the cracks.

“If the iron bars are exposed to water, they become acidified and expand, which leads to more cracks in the concrete,” he said.

Sewer pipes in Japan are built to last 50 years, but 7% of the 490,000 kilometers of pipes across Japan were past that period as of March 2023, according to the land ministry. Some 40% of the pipes are expected to be more than 50 years old in 20 years.

Tanikawa argues that major drainage pipes will need to be built to last longer to prevent pipe ruptures like the one in Yashio. Pipe lining with vinyl and frequent cleaning can prevent decay, too, he said.