In the "speed of light" shrine. To get the hidden chest at the end, you need to get into the final room while the water is up, but the switch to open the final door requires the water to be down. You have a narrow window of 5-10 seconds to get into the last room while the water is rising. You're supposed to use a remote bomb to trigger the switch to raise the water while you're inside the last room, but I didn't figure that out.
Instead, I used an ice block to jump onto one of the torch sconces on the wall by the edge of the water, then hit the switch with an arrow, quickly jumping off and gliding through the door and into the final room in that 5-10 second window.