Jamsil Indoor Swimming Pool, which opened all the way back in 1979, hosted the swimming competition of the 1988 Summer Olympics and nowadays hosts national and local swimming competitions, as well as swimming classes for Seoul locals. People can also pop in just to swim while there's a lifeguard on duty. However, in order to use the pool you have to become a member of the facility. You can fill out an application and submit it online or to the office at the pool along with the monthly fee of 80,000 won, although the application is all in Korean. You can find the application at the bottom of the page here.