As recently as 2013, the Schmidt Ocean Institute plainly stated: "... we aren’t even close to having fully mapped [Earth's] seafloor." In fact, according to NASA, only between 5 to 15 percent of the ocean's depths had been surveyed by traditional sonar techniques at that point. That's because it's expensive and time-consuming to scan the bottom of the ocean. In most cases the scans were done in places where ships travel, because we needed to know what the ships were traveling over. Popular shipping routes have been covered, as have near-shore depths, but that's about it.
Yet we've all seen those maps of the Earth that detail all kinds of subsurface ocean features. Where do those maps come from? Well, it's really a question of scale; we know where most of the largest underwater mountains and valleys are, but in most areas of the ocean, we don't have much detail beyond that. So from the distance perspective of a globe, sure, the seamounts and deepest depths are known, but get closer and it becomes much fuzzier. Basically, we've had a low-res view of the ocean floor.
Just last year, NASA was finally able to "see" below the ocean waves in much finer detail than ever before. Instead of using sonar, NASA mapped the ocean floor by examining the shape and gravity fields of the planet, called geodesy.
According to the NASA Earth Observatory: (This link offers a closer view of the map above.)
"David Sandwell of the Scripps Institution of Oceanography and Walter Smith of the National Oceanic and Atmospheric Administration have spent much of the past 25 years negotiating with military agencies and satellite operators to allow them access to measurements of the Earth’s gravity field and sea surface heights. The result of their efforts is a global data set that tells where the ridges and valleys are by showing where the planet’s gravity field varies."
How to see what really lies beneath
Geodesy works for seafloor mapping because mountains underwater (like those above) have huge amounts of mass that exert a gravitational pull on the water around it, which causes water to pile up in those places. Yes, there are "bumps" on the surface of the ocean, which can vary as much as 200 meters in height. The same thing goes for the reverse, when it comes to massive valleys, or even smaller features.
The video above explains how geodesy works, from its earliest beginnings until current day. You can skip to 1:45 to get a visual of how satellites are used to measure gravity and sea-height.
Satellites are still used in this type of mapping, but unlike terrestrial mapping, where images are used along with existing information, in this case altimeter (height) measurements from satellites CryoSat-2 and Jason-1 of the sea surface were combined with existing data to understand deep-ocean features, some of which were covered in silt and not "visible" anyway. Again, these are differences in sea height caused by gravity, not the physicality of the features themselves.
Lots of new underwater details were found when this new map was created, with any feature larger than 5 kilometers now included on the map — about twice as clear as before. As reported in the journal Science, "previously unknown tectonic features, including extinct spreading ridges in the Gulf of Mexico and numerous uncharted seamounts," were detected.
But even with these new ocean maps, we still know more details about the surface of Mars. The red planet has been carefully mapped by orbiting satellites over the past 15 years; its map resolution is 20 meters (66 feet). But the ocean's resolution with the new maps detailed above is at best about 5 kilometers (or 3.1 miles).
It's amazing to think that new features of our own planet are still being discovered. And it's none too soon, as deep-sea exploration is accelerating, with China making an almost-10,000-foot deep-sea lab in the South China Sea a near-future priority. (Most are assuming the country is investing in such a structure to extract minerals from the Earth's crust). Higher-resolution sonar models will continue to be made of the seafloor, but humans may well land on Mars before we have as detailed a map of the ocean floor as we do right now of Mars.