(CNN) -- The construction of what would be Seoul's tallest building has hit a snag following the appearance of a number of mysterious sinkholes in and around the neighborhood where it is being built.

Residents of South Korea's capital are alarmed by the increasing incidence of sinkholes -- depressions, measuring up to several feet in depth and diameter, which have suddenly appeared around Seoul.

The first one was discovered in June, and since then several others have appeared, local media have reported. Now, in a climate marked by increased safety concerns, the construction of a huge skyscraper in the city has come under scrutiny.

Last month, two holes within a couple of meters of each other were discovered near the National Assembly in the Yeouido district of the capital.

Several examples of this unexplained phenomenon have occurred near the site of the Lotte World Tower, a half-constructed commercial and office development by Korean conglomerate Lotte which, when completed, will be the sixth tallest tower in the world.

Holes have appeared in at least two streets in the Songpa district, where the new tower is being built, including a 50 centimeter (20 inch) wide hole only half a kilometer (a third of a mile) from the Lotte World Tower construction site.

So far, authorities are baffled. "We do not know the cause yet," a police officer told the Korea Times. "In cooperation with Seoul Metro and Seoul Metropolitan Government, we investigated the problem and only found that the holes have nothing to do with sewerage."

A lake near the site, which appears to be shrinking, is also cause for concern. Water levels have fallen about 70 centimeters (27 inches), although Seulki Lee, a spokesperson for Lotte Group, told CNN that it would be "nearly impossible" for the water to drain into the tower's foundations due to a slurry wall between the lake and the construction.

The spokesperson also said that the company was looking into the mysterious sinkholes.

"We are working on an investigation of sinkholes but it will take some time to figure out what's going on," she said.

She said that academics and engineers from Lotte have told the construction company that the sinkholes are not related to the site but it is "necessary to figure out what is going on" to provide assurance for the public.

Plans for the 555-meter (1,821-foot), 123-story tower were first put forward almost two decades ago, but planning permission was slow in coming, due to security concerns from a nearby military base.

Construction of the tower, designed by American firm KPF, is underway and more than half of the tower's floors have been completed. The architecture firm was, at the time of press, unavailable for comment.

Professor Hong Gun Park of Seoul National University's Department of Architecture and Architectural Engineering was a consultant on the project, completing an "outsider's evaluation."

He told CNN that the foundations of the building were solid, and that Seoul had no history of subsidence.

"People found sinkholes near the building site, (and) since there is a small lake, they are worried about the robustness of the foundation of the building.

"However, recently many sinkholes were found here and there in Seoul. Furthermore the foundation of the building is deep and is sitting on the deep hard rock. Thus in my opinion there is no problem (with) the structural safety of the building."

He said that it was unlikely that the Seoul municipal government would halt construction without reasonable cause.

The safety concerns over the building come months after the country was shocked by the sinking in April of the Sewol ferry, which led to widespread criticism throughout South Korea that safety was not a priority. Almost 300 people, mostly teenagers on a school trip, died when the ferry capsized. orv

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