Originally Posted by captain456
The secret cave in the Marine resort is a bit tricky to find. To find it, you first have to be familiar with the ATV park (I believe it's called Wild Hills Off-Road Park) and you need an ATV. The park is pretty huge, but one section of the track is split up by a small rock wall even though both paths lead to the same huge jump. If you've found that then head in the OPPOSITE direction of the huge jump. You'll find another ramp and to the right of it will be a huge rock wall. This wall will have a section that is smaller than the rest of it and a person will be standing on it. You have to jump off of the ramp and over the gap in the wall.
When looking for this part, I found it helpful to bring a dog along. The reason for this is that when you get to the wall with the people standing on it, my pup stopped in the middle of the road and started barking furiously. She also had an exclamation mark over her head. If not for her, I would have drove right past the wall! Thanks captain456 for posting this! It really helped me out.