Jet Force Gemini/Mission 5: Cerulean

Cerulean, a dusty planet compromised entirely of blue sand and caves...and the second place where Vela goes to in her quest to reach Ichor and find Lupus. This planet is unlike any you'll ever see for a couple of reasons. The majority of this planet takes place in cave and underground tunnels, there is a lot of running back and forth to finally get to your ship, and overall, the entire place is like a breeding ground of sorts for Drones. However, there are a few bonuses you can snag while on this planet: Fishface (remember him? The pervy Mining Mole Brother?) is here, waiting for you to conduct some "business", and you get two big bug-busters as well: The Tri-Rocket Launcher and the Homing Missiles. Unfortunately, this planet is a little on the long side, with lots of going back and forth, but it is worth it. Furthemore, you can rescue all the Tribals on this planet and save yourself from wasting time to come back. All in all, there is a lot to be done, so prepare your trusty machine gun and get ready to send some Drones to the grave.

Cerulean: Dune
This is the only area of the planet that you'll explore and also the only one that exists. Although its name hints at above-ground fighting, in reality, you end up going deep underground before surfacing again to get to your ship.