Flip a bike with a sidecar and land it so the sidecar is clipped through the floor. Helped to flip it next to a wall/building. Then Ctrl + 3 to move to the sidecar and you should be able to get out and clip through the floor. Can do it fairly consistantly, just hard to find a sidecar buggie. You fall into the water under the map and can swim to the centre of the circles.
The circles still damage you, but if you don't take any damage before you clip in it's likely you'll place first, if not guaranteed second.