Roads crack open, water gushes up: Heavy rains expose danger lurking beneath Seoul's streets

A green cover stretched across a sloping alley in a Guro-gu residential neighborhood on Friday, hiding the scar left by a road crack that had opened days earlier. Around the cover, a hole roughly the size of a car tire gaped in the pavement, and the asphalt edges curled upward, lifted from the ground. Residents said muddy water had gushed continuously through the cracks when the incident occurred, streaming down the slope and pooling throughout the alley.

The affected stretch runs through a densely packed multi-family housing block. A sloped road extends about 40 meters below the crack point, flanked on both sides by six residential buildings. Some units sit below street level in semi-basement floors, raising fears of flooding should water flow down the incline.

Residents who witnessed the incident recalled the alarming scene vividly. A neighbor surnamed Han said she had stepped outside to run an errand on Tuesday when she found water bubbling and surging up through the cracks. "The smell was so bad you could notice it even from inside the house," she said.

Another resident, Bang, 67, said the flow started as a trickle before turning into a torrent within a day. "Water was gushing from multiple spots, not just one, and we had to dodge it for three days," he said. He added that a similar incident had occurred two years ago and prompted repairs, and warned that patching a single spot would not be enough this time. "The whole area needs to be overhauled," he said.

Guro-gu authorities attributed the incident to a burst aging drainage pipe. A pipe that carries collected water out of a sump had ruptured, allowing water to seep beneath the road surface. The pressure generated in the process cracked the asphalt and forced sections of it upward.

A Guro-gu district office official said the monsoon rains were not the direct cause of the road damage, nor was the ground itself inherently weak. "We believe a drainage pipe that has been in place for more than 15 years was partially damaged by repeated shocks and pressure from the pumping process," the official said. He added that the incident was not being classified as a sinkhole or ground subsidence at this stage. The district plans to repave the road after the monsoon season ends and will secure a budget to replace the entire pipeline next year.

What may appear on the surface to be a simple road crack or water leak could in fact signal underground instability during the monsoon season, experts say. When soil is washed away underground, voids can form, potentially leading to further subsidence and eventually a large-scale sinkhole.

Sinkhole incidents have recurred in recent years. In 2024, a road suddenly collapsed in Yeonhui-dong, Seodaemun-gu, Seoul, injuring a driver and others when a vehicle fell into the hole. In March this year, a sinkhole opened in Myeongil-dong, Gangdong-gu, Seoul, killing one motorcycle rider and injuring a car driver.

The timing is particularly concerning. The Korea Meteorological Administration recently announced that the entire country has entered the full monsoon season. On ground already weakened by earlier rains, heavy downpours can accelerate soil erosion and raise the risk of additional subsidence.

Heavy rain accompanied by thunder and lightning was forecast for central regions and the Jeolla area from Tuesday night through early Wednesday morning, and downpours continued through that period. Forecast rainfall for Wednesday includes 5–20 millimeters for Seoul, Incheon, Gyeonggi Province, inland and mountainous areas of Gangwon Province, Daejeon, Sejong and South Chungcheong Province, and North Chungcheong Province; 5–40 mm for North Jeolla Province; 5–10 mm for Gwangju, South Jeolla Province, Busan, Ulsan and South Gyeongsang Province, and Jeju Island; 5–30 mm for Daegu and North Gyeongsang Province; and less than 5 mm along the Gangwon coast on the East Sea.

Ground subsidence cases are in fact concentrated in summer. According to the Ministry of Land, Infrastructure and Transport's Underground Safety Information System, summer (June through August) recorded the highest number of ground subsidence incidents nationwide, at 568 cases. Spring followed with 253 cases, autumn with 132 and winter with 93.

In response, the Seoul Metropolitan Government conducted special safety inspections at 32 excavation sites last month to prevent monsoon-season ground subsidence. Earlier, the city had also carried out joint safety inspections at 140 excavation sites deeper than 10 meters, beginning in March.

Park, the institute director, said that if one side of a road has sunk while the other has risen, soil underground may have been washed toward the sump. "It is difficult to explain all the phenomena we are seeing simply by a pipe leaking water," he said.

He went on to say that during the monsoon season, heavy inflows carry not just water but soil as well. "When muddy water comes out, it means sediment is mixed in and being lost," he said, warning that continued soil erosion could trigger further ground subsidence and potentially lead to a larger-scale sinkhole.