Black dragon pool gave a good impression, first it`s the pool`s water, very clear and you can see the deep of the bottom, and if you stand upon the bridge you can see the pool was separated into two half.  It looks like a eight trigrams since Black dragon pool was a Taoism.
Second, in ancient time, public think there is a king of dragon who lives here, he has the power of controlling the water and the weather, people respect this pool and wish the Dragon king will bring them good weather and clean water.