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Silent, hidden and deadly: Sinkholes erupt in Seoul as heavy rains, aging infrastructure wreak underground havoc

Published: 01 Aug. 2025, 07:00 Updated: 01 Aug. 2025, 15:21

LEE SOO-JUNG
[email protected]

A massive sinkhole is seen near an intersection in Gangdong District in eastern Seoul on March 25. The sinkhole appeared a day before and injured one vehicle passenger and killed another motorcyclist. [NEWS1]

How safe is the ground I walk on? Pedestrians in Seoul may be forced to ponder this question, given the recent spate of sinkholes erupting in the heart of the capital.

You might be treading on nothing more than a thin layer of soil atop a cavernous subterranean void waiting to swallow you up. Indeed, such was the case in two recent sinkhole cases.

Last week, 36 people had to evacuate after a 2.5-meter-deep (8.2-feet) underground void emerged at a construction site in Dongdaemun District in eastern Seoul. An adjacent building was also affected by the land subsidence, and a window was shattered.

Three days later, another 1.2-meter-deep, 50-centimeter (19-inch) wide sinkhole appeared in Myeong-dong in central Seoul on a road where countless cars and buses pass every day.

A series of sinkholes and land subsidence cases have surfaced in Seoul in recent weeks amid the sizzling season and heavy downpours, a reminder of their invisible and unpredictable nature.

At times, they can even be deadly.

In March, a sinkhole in Gangdong District in eastern Seoul swallowed a motorcyclist, killing him. In August last year, a road in Seodaemun District in western Seoul caved in and trapped an SUV. The driver and a passenger were injured.

Despite efforts by authorities to better track and prevent sinkholes, the Seoul Metropolitan Government reported that 72 cases of sinkhole and land subsidence had occurred in the city during the first half of this year alone.

And sinkholes and cases of land subsidence — many of which appear out of nowhere — are likely to become more frequent in Seoul, an especially vulnerable area because of aging infrastructure, geotechnical experts say.

Vulnerable Seoul

Road safety personnel and an excavator conduct repair work where a sinkhole appeared in Seodaemun District in western Seoul in August last year. The sinkhole swallowed an SUV vehicle with two passengers. [NEWS1]

Of Seoul’s 25 districts, 21 experienced one or more cases of land subsidence this year, indicating that practically nowhere in the city is safe. Only the districts of Dobong, Yongsan, Yeongdeungpo and Seongdong saw no sinkholes or land subsidence.

The city government counts land subsidence if an underground void is 0.8 square meters or larger, or when its depth is 80 centimeters or deeper, or when there are deaths, disappearances or injuries related to the sinkhole.

Land subsidence is a phenomenon where land gradually or suddenly sinks or settles downward. A sinkhole, a type of the land subsidence, refers to an underground void that opened on the surface.

Left: Land subsidence and sinkhole cases in Seoul this year between January and June. Right: Monthly land subsidence and sinkhole cases nationwide from 2018 to 2024 [LEE JEONG-MIN]

Despite such frequent sinkholes, no fully transparent data or maps indicating land subsidence risk areas exist. Only a partial guide is available from the city authorities.

The Seoul Metropolitan Government decided not to disclose its 2024 underground safety map, citing accuracy issues. Instead, in June this year, the city unveiled another map showing the results of ground penetrating radar (GPR) — a technology that detects voids up to two meters below the surface.

A map available on the Seoul Metropolitan Government's website shows GPR results in key streets in Seoul. [SCREEN CAPTURE]

However, the map appears to be incomplete. It only surveyed 364 sites on some 350 kilometers of road — mainly sites where excavations and subway construction work were conducted. On the map, areas that were assessed to have underground voids are colored purple, while streets free of such voids are shown in blue.

However, an alternative map shows how underground soil across the capital is susceptible to land subsidence and sinkholes.

Several touristy neighborhoods in Seoul — the Euljiro and Myeong-dong areas spanning the Jung and Jongno districts, the Hongdae area in Mapo District in western Seoul, and the Sinsa and Apgujeong neighborhoods in Gangnam District in southern Seoul — were found to be especially risky, according to the Korea Association of Underground Safety and local newspaper Dong-A Ilbo.

The two organizations also classified nearly all neighborhoods along the Han River as grade 4 or 5 — the riskiest segments of the five-tier scale. The only exceptions were four neighborhoods: Hannam-dong in Yongsan District, Sangam-dong and Sinsu-dong in Mapo District and Heukseok-dong in Dongjak District.

Why is the summer season riskier?

A vehicle is partially swallowed by a sinkhole appeared in Dangjin, South Chungcheong in September 2021. The region received 83.5 millimeters of rain for two days before the sinkhole appearance. [YONHAP]

Of 1,398 sinkholes and cases of land subsidence nationwide in the past seven years, 918 happened between May and September, according to the Ministry of Land, Infrastructure and Transport.

Sinkholes are especially prevalent in summer as a result of the heavy rain.

Underground water sustains soil stability by filling gaps between soil particles that support structures buried beneath the surface, such as pipes. The water is meant to travel through the underground space slowly.

However, summertime heavy rain drastically increases the volume of underground water in a short period. Then, the water moves rapidly through the soil, carrying away the soil supporting underground pipes at an accelerated speed. Consequently, a void occurs. With no soil protecting and surrounding the pipes, they become susceptible to ruptures.

The recent sinkhole in Myeong-dong was a notable example. It happened about a week after some 70 millimeters of daily rainfall battered the capital for three days. An official from the city’s road safety authority said the “soil went into a broken part of an unused pipe and triggered the sinkhole.”

Seoul Mayor Oh Se-hoon inspects an underground construction site in Gangnam District in southern Seoul to prevent occurrences of land subsidence in April this year. [JOINT PRESS CORPS]

Prof. Baek Sung-ha from Hankyong National University’s School of Civil & Environmental Engineering said old underground pipes that have exceeded their life span — usually 30 to 40 years — are one cause of land subsidence and sinkholes.

Among this year’s sinkhole and land subsidence occurrences in Seoul, 18 cases happened due to underground pipe damage. Of the city’s 10,866 kilometers of pipes, 55.5 percent have been in use for 30 years or more.

By districts this year, Gangnam District in southern Seoul recorded the greatest number of land subsidence cases at 13. Songpa District, located next to Gangnam District, followed with 10 cases. Gangdong District in eastern Seoul recorded five cases, and the districts of Jongno, Dongdaemun and Nowon recorded four.

While Gangnam is seen as a financial, educational and pop culture hub of Seoul today, the neighborhood was rural farmland before city-led mass development began in the late 1960s and early 1970s. The concentration of skyscrapers can also undermine the soil stability as their construction pumps out underground water.

Baek told the paper that the risk of land subsidence will inevitably increase as underground development becomes more prevalent. “Excavation works during the construction of subways and skyscrapers can cause large-scale land subsidence and sinkholes,” he said.

More technologies, better safety

A Seoul city official inspects a device detecting underground void through ground penetrating radar in downtown city in April. [NEWS1]

Experts said robotic technologies should be used with GPR to detect underground voids preemptively and effectively.

Four-legged walking robots can overcome the limits of human-operated detection methods such as GPR, said Prof. Lee Jong-sub of Korea University’s School of Civil, Environmental and Architectural Engineering.

“In Europe and some advanced countries, four-legged robots are already in use to collect 3-D data and monitor risky areas that humans cannot access, especially in large construction sites and complicated terrain,” he said.

Prof. Lee also recommended utilizing data from interferometric synthetic aperture radar (InSAR) satellites and flying drones equipped with sensors that detect infrared light, enabling road authorities to identify signals or cracks on the ground and in soil in real-time. The InSAR techniques show topography changes through comparison of the reflective energy from a certain surface on Earth measured at different times.

District officials and safety personnel conduct recovery effort to fill up a sinkhole appeared in Mapo District in western Seoul in April. [NEWS1]

Prof. Baek urged the development of more sophisticated technology for underground development, adding that it is difficult to see GPR as a “complete technology.” He said there were several instances where the radar falsely suggested voids or failed to detect them.

“Society’s demands for underground development now exceed the capabilities of contemporary technology,” Baek said. “We need more precise construction methodologies and monitoring capabilities to cater to the development of complex and large-scale underground spaces.”

“The United States and China run monitoring systems using synthetic aperture radar (SAR) satellite footage that measure land subsidence in urban areas over the long term,” Baek said. “However, no such systems exist in Korea.”

BY LEE SOO-JUNG [[email protected]]