How Deep Is the Ocean, Really?
The short answer
About 3.7 km (2.3 miles) on average. A 2010 estimate puts it at 3,682 m. The deepest known spot is the Challenger Deep, in the Mariana Trench: about 10,935 m, give or take 6 m. That is almost 11 km (nearly 7 miles), roughly three times the average.
Most of that water never sees the Sun. Below about 1,000 m (3,300 ft) there is no sunlight at all, and roughly three quarters of the ocean's water sits below that line.
Where the sunlight goes
Sunlight is a mix of colors, and water absorbs them at different rates. Red goes first, close to the surface. Orange and yellow follow. Blue travels farthest, which is why deep water looks blue. Even in clear tropical water, only about 1% of the light reaches 100 m, and what is left is mostly blue.
Ocean scientists divide the water into zones by light. The sunlight zone runs from the surface to 200 m. From 200 to 1,000 m is the twilight zone, where only a faint glow gets through. Below about 1,000 m, light from above is gone. The only light left there is made by the animals themselves.
The ocean averages 3,682 m deep, so most of the water in a typical spot lies under that line. If you work it through with the ocean's area and volume, it comes to roughly three quarters of all the water.
Why a brighter Sun wouldn't help
Water does not take away a fixed amount of light per meter. It takes away a fixed share. In the clearest ocean water, each meter removes roughly 2 to 3% of the blue light that reaches it. Start with 10 times more light and you still lose the same share at every meter.
So brighter light buys only a little extra depth. It takes about 75 to 135 m of the clearest water to cut light to a tenth. A Sun 10 times brighter would reach that much deeper and no more. The film rounds this to 100 m. Each further 75 to 135 m would need another tenfold boost.
Who lives in the dark
The fish in the film is a Mariana snailfish, Pseudoliparis swirei. It lives about 6,200 to 8,200 m down, usually between 7,000 and 8,000 m. When scientists described it as a new species in 2017, it was the deepest fish ever collected from the ocean floor.
The film puts her at 7 km (4.3 miles) down, about 6 km below the last of the sunlight. In the film, the Sun's stretching arm gives up at 6 km, still a kilometer above her.
Key numbers
| What | Number | Source |
|---|---|---|
| Average depth of the ocean | 3,682 m (about 3.7 km, 2.3 miles) | Charette & Smith 2010, Oceanography |
| Deepest known point, the Challenger Deep | about 10,935 m, give or take 6 m (almost 11 km, nearly 7 miles) | Greenaway et al. 2021, Deep Sea Research I (NOAA repository) |
| Challenger Deep compared with the average depth | about 3 times deeper | Our arithmetic from Greenaway et al. 2021, Deep Sea Research I (NOAA repository) and Charette & Smith 2010, Oceanography |
| Light reaching 100 m in clear tropical water | about 1% of visible light, mostly blue | University of Hawaiʻi, Exploring Our Fluid Earth, Light in the Ocean |
| Bottom of the sunlight zone | 200 m | NOAA JetStream, Layers of the Ocean |
| Twilight zone, with only faint light from above | 200 to 1,000 m | NOAA JetStream, Layers of the Ocean |
| Depth below which no sunlight arrives | about 1,000 m (3,300 ft) | NOAA JetStream, Layers of the Ocean |
| Share of the ocean's water below 1,000 m | roughly three quarters | Our arithmetic from Charette & Smith 2010, Oceanography |
| Blue light lost per meter in the clearest ocean water | roughly 2 to 3% | Our arithmetic from Morel et al. 2007, Limnology and Oceanography |
| Extra depth reached by light 10 times brighter | roughly 75 to 135 m | Our arithmetic from Morel et al. 2007, Limnology and Oceanography |
| Where the Mariana snailfish lives | about 6,200 to 8,200 m, usually 7,000 to 8,000 m | FishBase |
| Year the Mariana snailfish was described, as the deepest fish ever collected | 2017 | University of Hawaiʻi News, 2017 |
What the film simplifies
- The film says "about four kilometers" for the average. Charette & Smith's 2010 estimate is 3,682 m, which is closer to 3.7 km.
- The 10, 40 and 100 m tags on the colors are rough marks. Colors fade gradually, and how deep each one reaches depends on how clear the water is.
- "Three quarters of the ocean" means three quarters of the water by volume. It is our estimate from the ocean's average depth, area and volume, not a measured figure.
- Below 1,000 m the film draws everything pitch black. No sunlight gets there, but some deep sea animals make their own light, and the film leaves that out.
- The Sun's glowing arm, its stretch and its bang are cartoon. So is the light on the fish: snailfish don't make light, the beam is our flashlight, and outside it she is drawn dark, apart from a tiny pink dot in the opening shot.
- "She has never seen sunlight" is about this one fish, living 7 km down. Some snailfish species live in shallow water and do see the Sun.
For teachers
Runtime 1 minute 2 seconds · Topics: Ocean depth, Light in water, Deep sea life · Full script below
Ask your class
In the film, the Sun shines 10 times brighter, but its light gets only about 100 m deeper. Why?
Answer
Each meter of water removes the same share of the light, however bright it starts. Ten times more light only reaches about another 75 to 135 m in the clearest water.
The ocean averages 3,682 m deep, and the Challenger Deep is about 10,935 m. About how many times deeper than average is the Challenger Deep?
Answer
About 3 times.
Sunlight ends at about 1,000 m, and the Mariana snailfish usually lives 7,000 to 8,000 m down. How far below the last sunlight does it live?
Answer
About 6,000 to 7,000 m, or 6 to 7 km, below it.
The script
Host: The Sun is trying to say hi to a fish. Seven kilometers down.
Host: Problem is, water eats light.
Host: Red goes first. Then orange. Then yellow.
Host: Blue hangs on the longest.
Host: By one kilometer down, nothing. Not a speck of sunlight.
Host: Three quarters of the ocean is below this line.
Snailfish: Hello? Is somebody up there?
Host: So the Sun shines harder.
Host: Doesn't help. Water eats bright light just as fast.
Host: So it stretches.
Host: Down to the ocean floor. About four kilometers, on average.
Host: Then into a crack in it. The Mariana Trench.
Host: She's still deeper.
Host: Fine. We'll cheat. Lights on!
Host: Meet a snailfish. One of the deepest fish on Earth.
Host: She has never seen sunlight.
Snailfish: Whoa! What is that?
Host: A flashlight. Like the Sun, but tiny.
Snailfish: ...Who?
Host: The Sun! The biggest, brightest thing in the sky!
Snailfish: Is it nice?
Host: ...It's okay.
Host: Next, we find out what the bottom of the ocean does to your body. Follow!
Sources
- Red light fades first in seawater, and blue reaches deepest.NOAA Ocean Exploration, Light and Color fact sheet
- Below about 1,000 m, no sunlight reaches the ocean.NOAA JetStream, Layers of the Ocean
- Clear seawater dims light by a fixed share every meter.Morel et al. 2007, Limnology and Oceanography
- The ocean averages 3,682 m deep, a little under 4 km.Charette & Smith 2010, Oceanography
- The Challenger Deep is about 10,935 m deep.Greenaway et al. 2021, Deep Sea Research I (NOAA repository)
- Mariana snailfish usually live 7,000 to 8,000 m down.FishBase
- Even in clear tropical water, only about 1% of visible light, mostly blue, reaches 100 m.University of Hawaiʻi, Exploring Our Fluid Earth, Light in the Ocean
- The Mariana snailfish, described in Zootaxa in 2017, was the deepest fish ever collected from the ocean floor.University of Hawaiʻi News, 2017