Blood Appears Green Deep Underwater and Here’s Why

Forget everything you think you know about blood color! Sure, it’s usually red, but dive deep into the ocean and suddenly… wait for it… your blood turns green!

The ocean is a realm of mystery and wonder, often revealing phenomena that seem to defy our expectations. One such intriguing occurrence is the transformation of the color of human blood from its characteristic red to a surprising green when viewed underwater.

At the heart of this phenomenon lies the interaction between light and water. As light makes its descent from air to water, it undergoes a fascinating shift: because it travels faster in the air than in water, light bends at the water surface (refraction) before being scattered or absorbed by solid particles. This bending ushers it into a world where its journey becomes increasingly influenced by water molecules. 

As we delve deeper beneath the ocean’s surface, the intensity of sunlight diminishes, and with it, certain colors in the spectrum are absorbed more than others. Red light, with a longer wavelength, is absorbed more by water compared to shorter wavelengths like blue and green, which penetrate deeper and dominate the light reaching our eyes underwater.

In this video, you can follow how the red color of a diver’s rash guard and gloves diminishes as the depth increases.

When blood escapes into this underwater environment, the absence of red light means that the red color, which is typically reflected by the hemoglobin in our blood, is no longer visible. Instead, the green wavelengths, which are not absorbed as readily, are reflected back to our eyes, giving the blood a greenish hue. This effect is usually noticeable at depths of 30 to 50 feet and below, where the red spectrum of sunlight is significantly reduced.

This scientific explanation aligns with observations made by divers who have experienced injuries underwater, noting the green appearance of their blood. Such firsthand accounts, along with scientific studies, have contributed to our understanding of this underwater spectacle.

Of course, the clarity of the water also plays a role – the cleaner the water, the deeper light can travel before succumbing to absorption and scattering. While the amount of light reaching the abyss is significantly less, it’s not a complete blackout. Traces of light, especially the tenacious blue wavelengths, can reach depths exceeding hundreds of meters in crystal-clear waters.

Finally, yet another intriguing fact: the strong absorption of red light by water is actually the very reason why there are so many red plants, corals, and fish in the ocean. Red is readily produced by organisms, yet it becomes conspicuous to predators in white light. However, at a depth of 20 feet, red light is absent, causing red fish to appear black.

“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.”

The green appearance of human blood underwater is a great example of how environmental conditions can alter our perception of something as familiar as the color of blood. It is also a good reminder that the colors we perceive are the result of the complex interplay not only between our eyes and light, but also its transmitting medium.

Sources: 1, 2, 3