What color is your blood? If you said red, you are correct – most of the time. Under certain circumstances, such as underneath the ocean, it can appear to be green. The reason behind this is the same reason why fish deep down in the Twilight Zone of the oceans have evolved red coloring. 

First things first, here’s a look at the green color we’re talking about, courtesy of a scuba diver who got bit by a Moray eel at around 20 meters (65 feet) under the ocean.

The reason this happens is fairly simple. If you remember from school science, any object we see is because light has reflected off it and into our eyes. Red objects appear red because they absorb other spectrums of light, reflecting the red light.

That’s how it’s taught when you are younger. But reality is a little messier, with objects generally reflecting a range of wavelengths of light. Blood absorbs most colors, and reflects back light mainly in the red spectrum. However, it also reflects a much smaller amount of green and blue light.

Water, meanwhile, appears to be slightly blue due to absorbing a lot of light in red wavelengths. If you go deep enough, enough red light is absorbed that only the green light is reflected back at your eyes (or recording equipment) so you perceive it as a green-blue color.

Because red light gets absorbed so well by water, a lot of fish living at depths are red.

“At depth, these animals are not visible,” the National Oceanic and Atmospheric Administration (NOAA) explains. “The black animals absorb all colors of light available and the red animals appear black as well since there is no red light to reflect and their bodies absorb all other available wavelengths of light. Thus, in the deep ocean, red and black animals predominate.”