The Mariana Trench is more than a mile deeper than Mount Everest is tall. Filmmaker James Cameron is planning to dive to the bottom and film it. National Geographic has released a couple of preview videos about the dive. The two videos, embedded below, provide a couple of quick lessons about the deepest valley on Earth.
Sounding the Bottom is a short explanation of how sonar is used to measure depths.
The Long Way Down offers a nice visual perspective of just how deep James Cameron will be diving.
H/T to The Adventure Blog.