 Complete the following phrase:  In late 1970s, a small islet in the northwest part of Seoul, called Nanjido, was made into a landfill due to the waste disposal problem that arose, due to the rapid development of Seoul into a metropolitan city. The park was once a 15-year-old landfill that consisted with 92 million tons of garbage, and it took 6 years to stabilize, and an extra year to build the park. The 