Everything in this photo is underwater. A mixture of sand and water flows across a platform submerged inside this tank. Observation windows allow a glimpse into this underwater world. The reflection from the water surface turns this world upside down and makes you question what's real.
This photo was taken during an experiment conducted to observe the flow dynamics of turbidity currents, which are the "rivers" of the sea-floor, transporting sediment from the shorelines into our deep ocean basins.