Do You Even Fathom?

If you’ve ever been in a boat with oars, have you ever stuck one of those into the water to see how far down it goes?

Today, if you want to get an idea of how deep the ocean’s floor is, oceanographers will use a variety of state-of-the-art technology. One way is to bounce sound waves off the ocean floor, then measure the time it takes for the sound to return. This is called sonar, and if you use multiple waves at a time, you can also map large areas of the ocean floor in a relatively short time.

Oceanographers and researchers might also use satellite altimetry, which relies on measuring the height of the ocean waves from space. This can cover a vast area in a short period of time, but it has limitations in detail, so sonar is better whenever you want more detail.

They can even send little autonomous vehicles down there, specifically designed to withstand the absolute crushing pressure underneath all that water. These little dudes can get the details filled in, especially in hard to “see” areas.

While the sort of precise mapping I’m describing here is very new, we humans have been trying to figure out how far the ocean goes down for as long as we’ve been getting into the water.

The method we used for thousands of years is incredibly simple, and it’s an idea anyone can instantly grasp today. You put a heavy thing at the end of a rope, and then you drop that anchor into the water. By measuring how much rope had passed, you can get a good idea of how far down the water goes.

This is just a fancy version of sticking that oar in to see if you can reach the bottom, but it served mariners incredibly well. Figuring out how far down the water went wasn’t just an exercise designed to satisfy one’s curiosity, either—it was often a matter of life and death. If a ship’s waters weren’t deep enough for a ship to traverse, that ship was very likely to sink.

Over time, this rope-and-anchor measuring trick became more and more precise, and people in England began to use a word to describe the measure. The easiest way to measure how much rope had gone by was by stretching your arms out, so you could determine how many of those arm-spans down it was.

In Old English, there was a word for an embrace called a fæthm. To the sailors out there, the measuring mariner must have looked like he was giving the rope one big hug after another as he measured the depths. This word gradually became the modern word fathom, and it became a standard measure of depth in the nautical world for centuries, endorsed by the British Navy.

It was only during the 20th century, as all these new imaging technologies began to spread, that the fathom began being replaced by the meter.

Even though the rope-and-anchor method of measurement dominated for thousands of years, there were some noteworthy innovations related to how far down things go. 250 years ago, Pierre-Simon Laplace determined the average depth of the ocean with an ingenious method. To find out how he did it, check this out next:

Or, read about our ancient seafaring ancestors who were unbelievably clever: