海洋到底有多深?

It’s often said we know more about the moon than we do about the depths of the ocean. There is a lot we don’t know about the H2O that covers much of the planet. Now we’re getting closer, though, to an important understanding: Just how deep are the oceans, and what’s the volume of all that water? The latest, best estimate is 1.332 billion cubic kilometers, according to research published in the journal Oceanography.

That’s actually lower than previous estimates by about five Gulf of Mexicos. It’s not that there’s less water out there. Rather, new satelite images have presented a clearer image of all the mountain ranges strewn across the ocean floor. Those peaks displace what we’d thought of as space for water.

But even the new satellite-based radar images still need fine-tuning. As researcher Walter Smith says, “We’re seeing only really big mountains, and in a blurry way. The resolution is 15 times worse than our maps of Mars and the moon.”

Ship-based sonar would help, but ship-based instruments have mapped only about 10 percent of the ocean floor so far. We really do know more about the surface of the moon than the total ocean floor.

—Cynthia Graber