When I saw the Caribbean Sea in person for the first time, my eyes metaphorically popped out of my head. As a kid who grew up in South Jersey, I was used to the dirty, almost brown, kinda-sorta blue color of the coastal Atlantic Ocean. But this was different. Staring at that bright, vibrant, and seemingly crystal-clear water, I had many questions. Where did that color come from? And why can I see my feet here, but not at home? Is the Caribbean water cleaner? Is the sun stronger down south? And how come it’s green-blue near the shore, yet navy blue a mile off shore?

Having traveled quite a bit since, I’ve heard all kinds of explanations from common folks, some chalking color differences up to pollution and others to salinity. While I’m certain that many factors, including those two, play some small role, the biggest influencers are the floor, depth, and microorganisms of the body of water.  

First off, let’s tackle why water, in most cases, appears blue to begin with.

Shedding a little light

If you’ve ever taken a cruise, you know that the farther offshore you sail, the deeper and bolder the blue becomes (navy blue). That’s because there are no reflections off the sea floor in very deep water, meaning that a majority of the sun’s rays are absorbed by the water itself. Water molecules, by nature, absorb reds, greens, oranges, and yellows, but spit out blue.

“When sunlight hits the ocean, some of the light is reflected back directly but most of it penetrates the ocean surface and interacts with the water molecules that it encounters,” explains NASA’s Oceanography Division. “The red, orange, yellow, and green wavelengths of light are absorbed so that the remaining light we see is composed of the shorter wavelength blues and violets.”

Sanding Off

As the water depth decreases and the light is able to penetrate all the way to the bottom, the makeup of the floor becomes a factor in determining water color. For example, the coarse Caribbean coral is going to reflect light differently than the fine sand found in the Northeast. These differences in absorption and reflection affect visibility as well as color.

Whatever light is not reflected back from the top layer of water or the bottom of the sea floor is absorbed by something in the water. As we saw above, lots of light is consumed by the water molecules themselves, but microorganisms living in the water also “eat” their fair share. The final major players in determining color are the particles and organisms found and suspended in the water. Phytoplankton, for example, harbors chlorophyll that absorbs red and blue light and reflects green. If a high concentration exists in one area, the water will take on a green hue. The more there are, the greener the water will appear.

Those three factors—depth, floor makeup, and life (plus intangibles, like pollution, as mentioned above)—will interact to produce whatever color we happen to see. The same principles apply to other bodies of water, like lakes, craters, and rivers. It’s all about what’s in and under the water.

And, despite our focus on the oceans, it’s not all about being green, blue, or brown. Check out these uniquely colored tourist attractions found in different parts of the world as examples. If you thought the greenish-blue of the Caribbean was impressive, the red and black volcanic lakes should knock your socks off.

Laguna Colorada, Bolivia

Courtesy of Flickr user Valdiney Pimenta

Red sediments and algae pigmentation produce the unique red color of this salt lake in Bolivia, which is further contrasted by the white borax islands that are spotted throughout it. Located at more than 13,000 feet above sea level, the lagoon is part of the Andean Fauna National Reserve and is a common roosting spot for a variety of flamingo species.

Kelimutu Volcano, Indonesia

Courtesy of Flickr user NeilsPhotography

This volcano harbors three crater lakes at its summit that are strikingly different from one another in terms of color. Typically, Tiwu Ata Mbupu (Lake of Old People) appears blue, Tiwu Nuwa Muri Koo Fai (Lake of Young Men and Maidens) green, and Tiwu Ata Polo (Bewitched or Enchanted Lake) either black or red, although they all are known to change shades quite frequently and unpredictably. The latter two are separated by a crater wall, creating a stunning distinction when viewed side-by-side, especially when they are green and black, as seen in the photo. Thus far, research has revealed no official explanation for the differences and changing colors, but the general consensus is that chemical reactions are being triggered by volcanic gas activity that drives nutrient-rich water to the surface.

Lake Pukaki, New Zealand

Courtesy of Flickr user Peter Nijenhuis

Glacial erosion fills this body of water with glacier flour, or finely-ground rock particles, resulting in a frosty, cloudy-blue color (this mixture is sometimes referred to as glacial milk). Lake Pukaki has a surface area of approximately 111 square miles and was formed when glacial debris known as moraine dammed up the valley. There are glacier lakes in at least a dozen countries throughout the world that take on this “milky” appearance. While they are not abnormally colored, the Great Lakes are the largest glacial lakes in the world.