Question: How do you measure the depth of an ocean ?

Hi!
Well, there are a few techniques used for this.
Historically, before scientists developed new techniques, the depth of the ocean was actually measured by sending a rope with marks on it (all with the same distance between each other) from a ship to the bottom of the ocean and then counting how many marks had gone into the water. So a bit like using a ruler! Crazy, isn’t it?
Of course in most cases this wasn’t very accurate…

Now the most common way to measure depth is by using SONARS mounted on ships. These instruments send a sound wave from the surface to the bottom of the ocean. Then they use time that it takes for the sound wave to be transmitted back to calculate what the depth is. The instruments can do this because we know what is the speed of a sound wave when it’s travelling in the water.
Sometimes more sophisticated types of SONARs (echosounders) are used for specific very deep areas. For example, scientists used these to get an accurate mapping and depth estimate in the Mariana Trench (in the Pacific Ocean, which is the deepest point in the global ocean) and they concluded that the depth in the deepest point there was 10,994 meters (with an accuracy of plus or minus 40m, which is incredibly precise!).

Another way to measure depth is to use satellite imagery. This has been used to calculate the mean depth (and also the volume) of the entire ocean. The satellites that orbit in space can be used for this.
First, the scientists have measured the height of the satellite above the Earth’s surface (assuming that it’s perfectly flat, even if it is not).
Then, the satellite measures the height above the ocean surface (also in this case by sending out a pulse and measuring how long it takes for it to come back).
Finally, the difference between these two gives you the height of the ocean surface. These calculations are adjusted based on observations and measurements of tides and waves.

On a ship, apart from using the SONAR, the depth is also measured whenever an instrument called CTD (conductivity, temperature, depth) is sent straight down to the ocean floor. This instrument measures temperature and salinity and can also collect water samples while it’s coming back to the surface using big bottles. There are a couple of pictures of that on my profile, because we were using this instrument very often when I was on the research cruise in the North Atlantic.