Why is this mountain in Seoul filled with trash from the 70s??

Hi everyone😊
As you might know I love the mountains in Korea and I love to go on hikes and walks on a nearby mountain. But whenever I go there, I often see some litter on the ground.. It always leaves me feeling bad when I just ignore it. This has really encouraged me to pick up the litter I see around me whenever I can.
This time I went to clean up a place where I've seen a lot of trash before. However I came across something much older than I had expected..
Although I live in Seoul, I've seen many animals on a hike in the mountain. I've walked by a squirrel, weasel, mole, a salamander, and even a wild boar! :D I realized that even the ecosystem of a small mountain in Seoul is very diverse. To protect these ecosystems, we should work hard to keep our environment clean🌏🌏