70% of the Earth is ocean, but we've only explored about 5% of it. So more than 65% of the world is completely unexplored.

The ocean is really, really big. In fact, it's so big that we've barely even scratched the surface when it comes to exploring it. According to some estimates, we've only explored 5-7% of the ocean floor and less than .5% of the total volume of our bodies of water.

The primary reason why we've seen so little of our oceans is the extreme difficulty of getting to and exploring the deep ocean. Down in the depths, it is almost impossible to survive without specialized equipment designed to withstand extreme pressure and temperature differences.

As you go down deeper and deeper, the pressure in the ocean increases by about 1 atmosphere for every 10 meters of depth. That means that in the deepest parts of the sea, the pressure can get up to 1000 times the pressure at sea level!

(Source)