With the exploratory glamor being hogged by the space program, it is good to remind ourselves of the vast unexplored regions of the Earth.
Here is a nice graphic that shows what happens as we go down deep into the ocean. Of course, we don’t really know for sure since going deep is a formidable challenge. The deep ocean is largely an unexplored frontier.
I had not been aware until I saw this that two explorers Don Walsh and Jacques Piccard had in January 1960 actually gone down to the deepest point in the ocean, a place known as Challenger Deep which is at a depth of almost 11,000 meters in the Mariana Trench in the Pacific Ocean. They did it in the bathyscaphe Trieste. The round trip took about eight and a half hours, with only about 20 minutes spent at the bottom.
Surprisingly, no manned craft ever returned to that depth (though robotic vehicles have) so Walsh (now 80 years old) is the only living human ever to have reached the bottom of the ocean. The pressure down there is over a 1000 times atmospheric pressure and it was thought that fish could not survive at such pressures but Walsh and Piccard reported seeing small fish swimming around.
As the graphic below shows, the deepest part of the ocean is just a tiny fraction of the distance to the center of the Earth. (If you want to see the depth of each layer, see here. Note that the figure is not quite to scale.)
It is humbling to think that despite humans being around for millions of years, we have exhaustively explored just a tiny fraction of our own planet.