As one stands on a beach, looking out over the water as waves wash up on the sand, one's eye is drawn to the horizon. Above that faint border where the sky meets the sea, light is scattered and diffused by water vapor and patches of cloud. Below the horizon, some of the sun's light and heat is reflected by the surface of the sea, and some penetrates to a depth of several hundred feet. Still deeper, the only source of light is the bioluminescence of marine organisms, and near the bottom the water looks pitch-black. However, the deep layers of the ocean are not apparent to a person standing on the shore. From the shore, the eye sees only two planes: the plane of the sky meeting the plane of the surface of the sea. To visualize the sky and sea in deep layers of color and light requires an effort of imagination.
Cole TB. Brilliant Scape (Blue). JAMA. 2010;304(21):2328. doi:10.1001/jama.2010.1736