SUBMARINE CAR! VORTEX underwater glitch!
It's true! There are some caveats tho.. you are limited to the underwater space contained above the dam.
I'll re-state the instructions (deeply sorry, I hadn't thought of crediting the original poster so I don't remember
as of typing this.. google vortex submarine.)
Get the vortex near the bridge in Valle Occultado (north on the map, just west of center. it's at the water, near
the seaplane.) Then, do the unique jump off the cabin next to where the Vortex is. it takes some practice, but it
gets easy. The mode changes to cinematic and (I suspect because you haven't touched land yet, as long as you
land on the water) it stays in cinematic view. Now you can roam around freely and it stays in cinematic until you
touch ground. Now, using the minimap, navigate until you hit the dam at the right, near the end, but not too near.
The control rumbles and the cinematic sequence aborts. Release the throttle at this point until the view resets.
You will gradually start floating up to the surface. You can control your depth with the throttle. If you touch the
bottom OR the surface, your fun is over.
Here's what my guesses are about what's going on:
I think that the unique-jump cinematic mode doesn't finish until you actually touch land. I think that there is a bug
which sets you at game sea level when it resets, which would explain why this works in the water held by the dam,
because you end up submerged. Also, I think there's a bug which inverts your altitude variable, which would explain
why you invert if you touch the surface. The F16 fighter jet had a similar bug which would invert the instuments
when climbing from below sea level.