Right after you cleared the Highland Field, your next step should be to finish this cave- the last one in the Desert Oasis. With Blue Pikmin from the Mountain Springs you'll be able to drain the watering hole and scale the cliffs and venture through the wilderness plateaus at the far end of the area. At the final point, you'll reach the second of the Dark Caves. It takes you through a series of sprawling subterranean wastelands loaded with enemies, with the occasional rest stop. Get used to the sheer number of enemies here- this is a war. Speaking of which, you'll have plenty of encounters with enemies built like they were made for fighting, including the boss. Once you get its Dark Spore (assuming you found the one at the end of the Dark Field Cave) you will have opened the way to the nex