If the minibikes survived the fall you can just dismantle them to retrieve them, same for underwater too.
I remember my first game when I finally made the mini-bike. I was driving along happily, and then fell into a cavern.
I spent the entire even and most of the next day digging/mining (with stone shovels and stone axes) a path that I could climb/jump out of. It was by no means a path that the mini-bike could drive over.
At this point, I am near starvation and dehydration. I run back to my base to eat, drink, and load up on supplies to get my mini-bike out.
I return and dig/build a ramp out of the cavern for the mini-bike.
About 3 game days later, I figured out I could dismantle the mini-bike.
Lessons learned:
1. Mark caverns on your map. As you are driving along the icon will be on the compass at the top of the screen. When it is white (no gray), large, and dead ahead, be careful.
2. I always fine the caverns as multiple holes in a general area. Spend the few moment to mark them all.
3. Do the same with the little ponds/watering holes around the map.
4. If you have the time, build some wood block posts around the hole to set the edges so that you can zip by in the future.