Humans have been to every corner of the planet and built either an Internet café or a Starbucks in almost every city. You can find plastic in the middle of the ocean and Mt. Everest base camp is a vast rubbish heap. Satellites monitor virtually every square meter of Earth every day and Google has sent a camera down almost every back road.

So what else is there left to really discover?

In fact, there is a part of our planet the size of Australia that is almost entirely unexplored and unknown to humans. And from where I'm sitting it's only a couple of miles away—straight down.

The hadal zone, named for the Greek god of the underworld, begins somewhere around 6,000 meters below the surface and ends wherever life ceases to exist in the sediment at the bottom. For decades scientists presumed these depths were a desert in the middle of the seafloor. Nothing, they concluded, could survive in the frigid, pressure-filled, lightless, nutrient-poor world at the bottom of the ocean.

We know now that the deepest parts of the ocean are far from lifeless and may actually harbor as much diversity as coral reefs. But the fact remains that almost nothing is known about the hadal zone for the simple fact that it is so remote and so alien to life (and technology) adapted to conditions at the surface.

The entire ocean is like that, but the hadal depths have obscurity in spades. More than 98 percent of the ocean is above 6,500 meters depth, which we can reach with a fair number of human-occupied, remotely operated, and autonomous underwater vehicles. But it's that last 4.5 kilometers that have escaped observation, except via a few deep-sea lander systems that can observe a tiny patch of seafloor within reach of their lights.

That's why we're here. I'm with a group of 32 scientists, engineers and students on board the R/V Thomas G. Thompson, one of the largest ships in the U.S. oceanographic research fleet. With funding from the National Science Foundation, the research team will perform to perform the first systematic study of life in an ocean trench. Until now, the trenches have been terra incognita to scientific study. It's only in recent years that we've had the capability to study large expanses of the deepest 4,000 meters (2.5 miles) of the seafloor. When the Woods Hole Oceanographic Institution (where I work) developed the hybrid remotely operated vehicle Nereus, which we have on board, suddenly we gained the capability to move through the hadal environment; make careful, systematic observations; gather samples; and conduct experiments that will only begin to open the door to understanding how life has adapted to conditions there.

Yes, there was Trieste, and the Japanese remotely operated vehicle Kaiko, and James Cameron's DEEPSEA CHALLENGER, but the trips those vehicles made were mere pinpricks on the map of the hadal zone—and all focused mainly on the deepest spot in the ocean, Challenger Deep. As one of the scientists on board pointed out, "You don't climb Mt. Everest and then base all your assumptions of mountain ecology on what you find there." We want to make the first effort to build a picture of the hadal ecosystem that has evolved on Earth over millions of years and that has shared the planet with humans all this time without our knowledge, but presumably not without consequence for life in the rest of the ocean or on land.

We are currently about 150 miles northeast of Auckland, New Zealand, waiting to recover some instruments from the relatively tame depth of 4,000 meters so that we can turn them around and deploy them again.

Science is like that no matter how difficult your field site, so if we want to learn anything about the Hadal Zone, there will, over the coming six weeks, be a lot of repetition in what we do. And a lot of waiting. Here, it takes over an hour for our instruments to reach the seafloor and surface again—longer to do what they are designed to do. When we go deeper (our deepest site is over 10,000 meters, or more than 6 miles, down) it will take as much as 10 hours to reach the bottom.

Later today, Nereus will go in the water for its second dive of the cruise. After that, we have to deal with yet another reason why the ocean in general and the deep ocean in particular is vastly under-studied: weather. We have a storm coming, which will shut us down for a day or so, but which will give me the opportunity to catch you up on some of what we've already done.

For now, I'll leave you with one scientist's answer to my question about why he studies life in the hadal zone: "Because we don't know what's down there. Every time we go, we find something new."

And that is the driving force behind all true exploration.