Plain Zone

Meadows Leading to Adventure: The Plain Zone is the central area of the unnamed island Popola inhabits. It is the main hub for the Straw Hat Pirates in One Piece: Unlimited Adventure.

Geography
The Plain Zone is a grassy region of the game's unnamed island with average trees and sizeable elevation changes. Its most prominent features include a massive tree at its center that grows on two plateaus, leaving a path underneath it; a small island in the pond below a waterfall that leads to the Cave Zone; and a beachfront area where the Thousand Sunny could dock.

The zone's plateaus are only reachable by jumping on a bouncy mushroom, and treasure chests are hidden throughout it. The zone also leads to all other zones on the island except the Abyss Zone. While the Seaside Zone, Jungle Zone, and Ruins Zone are easily accessible from the Plain Zone's main area, the Cave Zone's entrance is surrounded by water. The Mountain Zone's entrance is at the bottom of a steep, winding hill, and the Dimension Zone's entrance is at the top of a tall, ascending stone path.

Camp
The Plain Zone was the home to the Straw Hat Pirates' Camp (キャンプ) while they stayed on the island. It was where they stored materials, cooked, built supplies, and enjoyed their downtime.

One Piece: Unlimited Adventure
When the Straw Hat Pirates arrived at the island and were separated, Luffy and some of the crew's belongings dropped down on the Plain Zone. Luffy began to explore the area, and Popola snuck behind him, hiding behind the bushes. After Luffy activated the orb and opened the path to the Seaside Zone, Popola jumped from the cliff above and struck him over the head.

Luffy left the Plain Zone and later returned with the rest of his crew after reuniting with them. The area became the crew's camp, and they returned there often while exploring the other zones. Franky built a bridge from the main area to the small island leading to the Cave Zone, allowing the crew to explore the island's bowels.

After the crew broke the seals in the Cave Zone, Popola decided to help them and accompanied them back to the camp. As they approached the Mountain Zone, apparitions of Portgas D. Ace, Shanks, Rob Lucci, and Kuzan stood at the entrance to warn them not to proceed.

The crew broke the remaining seals and confronted the Evil Guardian. After defeating him, the Abyss Zone and Cave Zone began to collapse, and Luffy used Gomu Gomu no Gigant Pistol to punch a hole in the cave's ceiling, allowing them all to escape to the Plain Zone above. They all got to safety, and when Nami asked Luffy if she could now have to orb to sell, it shattered in front of them. From it rose the ghost of Sealed, who only Luffy and Popola could see. The island's former protector thanked Luffy, and the crew left to sail off from the island. Before Sealed ascended to the afterlife, he left Popola with other creatures just like him to keep him company. The island completely transformed and became a paradise for him and his new friends.

Wildlife
Swallowtail Butterflies can be found on the Plain Zone's flowers, and Hercules Beetles and Golden Hercules can be found on the area's trees. Lovely Angels, Scissor Shrimp, and Adventure Fish can be caught in the zone's pond next to the Cave Zone's entrance. Island Clams, Adventure Fish, Panda Sharks, and Blue-Finned Elephant Tuna can be caught along the zone's beach.

Trivia

 * The Plain Zone was one of two of Unlimited Adventure's areas shown off in their early build stages, along with the Ruins Zone. In the early build, the area's foliage looked different, with more than one kind of tree and lower-quality textures.
 * The Plain Zone is the only zone in Unlimited Adventure with no enemies or bosses.