A 335-meter-long underground space located above Seoul Subway Line No. 2's tracks is seen during a media event, Tuesday. (Seoul Metropolitan Government)

An underground space located 13 meters below Seoul Plaza will be open to the public for the first time in 40 years, the Seoul Metropolitan Government said Tuesday.

The 335-meter-long underground space is located right above the tracks of the Line No. 2 subway and right below CitiStar Mall, the first underground mall built in 1967. It also lies in between City Hall Station on Lines No. 1 and 2, and Euljiro 1-ga Station on Line No. 2.

For what purpose the underground space was built is still unknown, but the city government speculates that it was built to connect City Hall Station and Eulljiro 1-ga Station in 1983.

The underground space will be open to the public as part of a tour program every Friday and Saturday from Sept. 8-23. The tour programs will run four times a day at 11 a.m., 1 p.m., 3 p.m. and 5 p.m., with each tour lasting one hour. For safety reasons, each tour will allow a maximum of 10 people.

A drainage system is also located above the underground space and, due to water dripping from the ceiling of the space, a stalagmite has been formed. Visitors will also be able to hear and feel the vibrations of the Line No. 2 subway during the tour.

A stalagmite has been formed in the newly unveiled underground space due to water dripping from a drainage system above. (Seoul Metropolitan Government)

All residents of South Korea are welcome to apply from Wednesday until 6 p.m. on Sep. 22 on the Seoul Public Service Reservation website. Reservations will be accepted on a first-come, first-served basis and spaces may run out before the end of the reservation period.

From Wednesday to Oct. 10, the Seoul Metropolitan Government will also be hosting a contest to come up with ideas for how the newly discovered space can be used.

Participants can submit their ideas in the form of images or videos and up to 35 idea submissions may be eligible for prizes. The participant receiving the grand prize can win up to 3 million won ($2,264) and their ideas will be actively reflected in the newly renovated space. Detailed information regarding the competition will be announced via the Seoul Metropolitan Government’s official website on Wednesday.

The project is part of Seoul Metropolitan Government's “Subway Station Innovation Project,” which aims to transform unused underground spaces in Seoul’s subway stations into various attractions that reflect local characteristics and trends. Besides City Hall Station, the initiative also seeks to remodel spaces near Line No. 5’s Yeouinaru Station, Line No. 2 and 6’s Sindang Station and Line No. 8’s Munjeong Station.

The newly unveiled underground space is shown in this architectural layout image. (Seoul Metropolitan Government)