I’ve been meaning to say something about this ever since Google Earth 5 was released earlier this month. Lots of people have discovered it on their own. The new Ocean and Mars features in GE 5 are really amazing. But, some people may find it a bit hard to navigate into the 3D world under the Ocean, or to fly around the surface of Mars. You could learn some of the cool mouse and keyboard tricks for Google Earth. Or, you could just turn on the built-in flight simulator and “fly” under the ocean or on the surface of Mars:

I’ve also created a short Tour
All you need to do to turn on the Flight Simulator is hit the keyboard shortcut: Ctrl-A (on Windows) or Command-A (on Mac). This will immediately put you in the seat of the F-16 simulation on Google Earth for your current view. You might also want to read these Flight Simulator tips which will help you fly. Quick tips: Click left mouse button once to turn on/off the “+” mouse cursor. When it shows the white “+” you are flying with the mouse. Keep the cursor near the middle of the GE screen to stay straight and level. You need to add throttle by pressing the “Page Up” key. In the Ocean, don’t hit the ground or you will crash. You can fly in the water or in the air though. If you “Switch to Mars“, you can do the same thing over the surface of Mars.
Here are some more tips for navigating in the Ocean by Googler John Gardiner.
I tried it and it works great!