In Seoul, South Korea, the Cheonggye Stream runs through the middle of the city. Over the centuries, as the city became more and more crowded, the stream and its many tributaries became very polluted. By the 1960s, part of the stream was so polluted that it was decided it would be best to bury it underground. After it was buried, a highway was built over it. This whole construction process took over 30 years.