Seoul city authorities are still facing a blank wall on what caused the portion of a sidewalk near Yongsan station to collapse, sending two people tumbling into a 3-meter sinkhole on Friday.

A local bus camera recorded the moment the sinkhole parted beneath two pedestrians after they got off a bus.

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The two were rushed to a nearby hospital and treated for scratches and minor injuries, South Korea's Yonhap News Agency reported.

The Yongsan Fire Department said the sinkhole opened up next to the site where Daewoo Engineering is constructing a two-building, 39-story apartment project.

City officials did not take any chances and asked the construction near the sinkhole to be stopped temporarily.

Authorities said Daewoo could be liable for flawed construction management, authorities said.

The Korean Geotechnical Society is also looking into the cause of the sinkhole while a metropolitan agency is examining the site for other possible sinkholes.

Sinkholes usually take thousands of years to take shape and differ vastly in size, experts say.

The deepest sinkhole is China's 662-meter Xiaozhai Tienkeng.

Egypt's Qattara Depression is about 80 kilometers by 121 kilometers in area. Usually, sinkholes are only a few meters in size.

There are indicators of possible sinkholes in cities. These include windows and doors failing to shut properly or cracks appearing in the support structure of houses.

In other cases, movement in the ground can be detected. Torrential rains or clogged drainpipes can lead to a collapse.

Experts say sinkholes are usually the result of what are scientifically known as Karst processes. Holes form when acidic water dissolves a sediment of rock beneath the ground.

The topsoil sinks into the cavity when the weight above it is too heavy. This layer is usually a soluble carbonate rock, such as chalk or limestone.

Usually, rainfall settles in the soil, taking in carbon dioxide and processed by decaying vegetation. This results in acidic water that settles in the soluble rock.