It’s called Cheonggyecheon, 청계천. Google Maps in Korea is really, really poor (for legal reasons). The satellite quality doesn’t even come close to capturing this (you can barely even tell it’s there), and Street View is just from the road (as of 2018 at the most recent) where you can see that it’s there, but not get much of a sense of it. There are a very small number of those individual non-path Street View photos.