The discovery of seven underground hollows in the vicinity of Seokchon-dong in southeastern Seoul has residents in the area fearful of the possibility that roads could cave in and buildings could collapse.

The hollows were discovered while the Seoul Metropolitan Government was inspecting the underground conditions in the area following the recent appearance of sinkholes there. The city, for now, has ruled out the construction of the 555-meter-tall Lotte World Tower as the cause of the hollows and sinkholes, pointing instead to the construction of Subway Line No. 9 as a possible culprit.

However, nothing has been confirmed and investigations are still underway. Meanwhile, Seoul City has embarked on a large-scale safety inspection of underground conditions in Yeouido through which the completed portion of Subway Line No. 9 runs.

The soft ground conditions of Yeouido are similar to that of Seokchon-dong and the recent appearance of two sinkholes on the road in front of the National Assembly building, under which Subway Line No. 9 runs, is a cause for concern. The subway tunnels in both Seokchon-dong and Yeouido were drilled employing a shield tunneling method using tunnel boring machines.

It was fortunate that the hollows in Seokchon-dong were discovered before a major catastrophe. The inspection of the Seokchon Underpass following the appearance of sinkholes in the area resulted in the discovery of several cracks in the underpass structure and its subsequent closure.

The Ministry of Land, Infrastructure and Transport is conducting safety inspections of 19 major construction sites across the country, including all sites of the Subway Line No. 9 third phase construction. The ministry is expected to release the results of its inspections early next month.

Modern cities of today would not be possible without their intricate networks of underground ducts, pipes and tunnels supplying water, electricity, gas, sewage and transportation services. Because this huge maze lies deep underground, we go about our day oblivious to what goes on under our feet.

It is only when problems surface ― flooding manholes, gushing water pipes, gas explosions and sinkholes ― that we are alerted to the potential dangers that lurk underneath. As a city ages, so do its many pipes and ducts. And as a city expands underground via subways and underground passes, engineers rely on the latest technology. All these are causes for concern, and the government should stay vigilant about what goes on underground.

The discovery of hollows was a stroke of luck. For safety, we cannot rely on luck alone.