This is a technical glitch, so I'll post a video to help you out.

Step 1:

Go to the area highlighted on the map in the opening of the video.

Step 2:

Scale up and down the rock shown in the video until you fall through. Move back some to fall off an invisible ledge allowing you to see.

Step 3:

Spawn a vehicle in the area shown and carefully follow the path shown as well. When you reach the second and last fall through area be sure not to press any buttons.

Effect:

In the end you and a vehicle are suspended in mid-air.

Heres the video:

Awesome glitch, I like to spawn a helicopter under there and then fly it around under the map.

Too much work for to little outcome. I thought you were going to just drive and keep floating. Now that would be worth the hassle.

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lol, yeah. I dont even post vids with glitches, but it was to hard to explain. Its easily tweaked and helps people get underwater with vehicles and shows a safe route.

It'd be better if you can stay up there and fly.