Beyond Good & Evil HD

As the description states, there are a total of four looters to chase inside caverns while in your Hovercraft, each called Looter's Cavern 1, 2, 3, and 4 respectively.

Here is a link to an overworld map of Hillys with the locations of each Looter's Cavern (credit to x MJ x on Gamefaqs.com for the map).
Looter's Cavern 1 - #9
Looter's Cavern 2 - #10
Looter's Cavern 3 - #11
Looter's Cavern 4 - #12
When you are about to enter these caverns you get a voice over warning that your are entering the area. The objective from here is simple, chase them down, avoid obstacles, and retrieve what was yours and then some.

Caverns 1 and 2 are easy to get to at the beginning of the game. Caverns 3 and 4 you will need the Jump Jets upgrade from Mammago's Garage, it costs 15 Pearls. With these, it'll allow you to jump the red laser barrier that is behind the lighthouse.

Chase Tips:
- Each chase gets harder than the previous but only really need your boost and jump to get through them.
- Each time you start, it'll take 100 credits from you and then the chase begins. If you fail, it'll restart back at the beginning where you entered.
- Avoid obstacles and try to not get shut in by the doors that close each time you come to an electrical box which is visible by a yellow current that travels along the wires to the door.
- If you feel you are struggling getting to the doors in times then have a few boosts handy to use.
- Each door has a number that counts down so you'll know when the chase is about the come to an end.
- Once you have the looter cornered blow them up to receive your credits and a pearl.

There ya go, congrats on an easy achievement!