I lived in Hilo for years, right up the road from Rainbow Falls. It's a tiny attraction, with maybe twenty car/bus stalls, a restroom, and the falls. The falls are part of the Wailuku River which originates on/near the slopes of Mauna Kea. After a hard rain, the falls can be unbelievably huge and roaring (and muddy). Try this trick: Stare at the falls in one place (try not to blink) without following the water downward for 30 seconds. Then quickly move your sight to the rock wall to the right of the falls. I won't give away what you'll see!