This is a large underground region. Unlike the previous area, you can explore nearly all of it without breaking out any barriors first. You don't even need to break down all the barriors to get all the parts here, but the shortcuts you create may save time in the long run.
The Blue onion is in this area, bur growing their population to a useful number will take some time. We will cover the onion and the easier parts here, and the more difficult parts in a separate page later in the walkthrough.
Space Float
We'll cover this first not because you should do it first but because the opportunity to get it could come at any time and you'll want to be ready. There's a creature wandering around this region which looks something like a dinner roll and is aptly named the breadbug. It makes a distinctive sound when it walks so the chances are you will hear it coming before you see it. Its one mission in life is to find stray food pellets and bring them back to its nest. It has a tough hide and Pikmin won't even bother to attack it, though you can damage it a small amount by throwing a Pikmin on top of it or by hitting it yourself. The problem is that it has somehow swallowed a ship part so you must defeat it some how. The trick is to wait for it grab a pellet and throw two Pikmin at it (one won't be enough). The Pikmin, carrying the pellet to an onion, will overpower the breadbug and when the pellet gets sucked into the ship the breadbug will nearly be killed. Either repeat, throw Pikmin on it or hit it yourself to finish it off and out will pop the Space Float. This is an optional part.
The Blue onion
You may notice the Blue onion sitting in a pool to the west of the ship, but before going over there it's a good idea to draw some Pikmin from the other onions and start them working on other things so they're not idle while you're trying grow Blue Pikmin. The most immediately useful things to do are to break down the lattice wall north of the ship and to build two bridges south of the ship.
Find the Blue onion; it's marked on the map. You know the drill by now; it turns bright Blue and ejects a seed when you approach and this time you get a Blue Pikmin when you pluck the sprout. From the fact that the onion is sitting in a pool of water you might guess that the special ability for this color is that they can survive in water. This may not seem like a big deal but there is a lot of water in the game so it's useful to have a large number of Blues on hand. In contrast, while the role of the Yellows is important, you only need to use a few at a time so there's not much use in trying to grow their number beyond what you probably have now.
Use the nearby pellet posies and the food pellet grow your Blues; you should be able to get to 16 from this.