The cylindrical tank containing the doped water and photomultiplier tube array is shown unfolded: The sides of the tank appear as a large rectangle, and the circular top and bottom arrays are shown as circles. Each filled color circle represents a photosensor that was hit by Cherenkov light in time with a neutrino beam, with the color scheme proportional to the amount of light (yellow is a high light level, purple is a low light level). Image: ANNIE collaboration