The Vast Plain
The Vast Plain is a location found in Wario Land 3 of the Wario Land series. It is the third of six levels on the north side of the Music Box World. At first the entrance to The Vast Plain is blocked, but it becomes accessible, along with The Peaceful Village, after Wario finds the Ax of Destruction from Out of the Woods and cuts down the tree.
The starting point of The Vast Plain lies in a prairie-like environment, with grass growing along the way and mountains in the background. Several holes in the ground lead to small underground caves. A door that can only be passed by Invisible Wario is located in the middle of the plain. Aside from the ground level, Wario can also explore the sky and the underground later. The sky above the plain is the habitat of two invulnerable enemies. At daytime, he can find Mr. Sun here, who tries to set him on fire. If he comes here at night, he faces Mr. Moon who shoots stunning stars at him. Below the plain lies a vast cavern lit by eerily burning torches. The place is also filled with guarded doors and Zombies. There are also some smaller caves, one filled with grindstone-wielding robots, one filled with Pneumos, and another one filled with water.
Treasuries
Grey Chest: The Colorless Mountain Slate 2 (right)
The Keystone Fragment can be found inside the Grey Treasure Chest, which is located all the way across The Vast Plain. To open it, Wario has to find the Grey Key, which is conveniently placed directly next to the chest. However, a security door separates the chest from the key, so Wario needs to be invisible to reach the key.
After Wario found both slates hidden in The Vast Plain and The Peaceful Village, he witnesses a cutscene. During the scene, the two fragments fly to the gate at the foot of a hill on the worldmap and form a stone tablet. This stone tablet then fits into the keyhole of the gate, which opens it. The gate then disappears, and Wario is allowed to access the Desert Ruins on the west side of the Music Box World.
Red Chest: The Seven Prismatic Colors Crayon-Blue
The Red Treasure Chest, which contains the Blue Crayon, can be found inside the underground cave filled with Pneumos. The player gets a chance to obtain it after equipping Wario with the Lead Overalls from The Volcano's Base. With his new Ground Pound move he can move the mole-like creature at the starting point of the level to uncover a Warp Pipe leading to said cave. The Chest and the Key are located in the cave, and Wario has to make use of his Puffy Wario ability, as well as to play a game of golf to reach both of them.
Once the Blue Crayon has been collected, it flies to a hill on the north side of the Music Box World. A building resembling a tower can be found there, complete with a door and a sign that reads "GAME". The scene is initially monochrome, but gains another color for each Crayon Wario manages to find. There are seven Crayons in total hidden throughout the game, and once Wario finds all of them, he will be able to play the golf minigame there at eny time the player desires. The blue Crayon colors the bushes that surround the building.
Green Chest: The Ring of Revival-Red
The Green Treasure chest with the red Ring of Revival is located in the sky above The Vast Plain. It gets within Wario's reach after he collects the Miracle Seed from The Big Bridge, which will cause vines to grow all over the place. One of the vines will take him to the cave that leads to the sky. There he has to evade the attacks of Mr. Sun and Mr. Moon, while collecting the Green Key and playing another game of golf.
The Red and the Blue Rings of Revival from The Vast Plain and The Pool of Rain respectively play a key role in accessing the Tower of Revival. Together they are able to summon the Tower to rise from the ground. After Wario puts on both rings and clicks them together, the tower appears on the south side of the Music Box World.
Blue Chest: The Red Bracelet
The Red Bracelet becomes reachable after Wario finds the two colored chemicals from The Steep Canyon and Beneath the Waves. Together they move the block which covers the entrance to the cave lit by torches. The cave holds both the Blue Treasure Chest, as well as its key. The many security doors in the cave must be overcome with Mad Scienstein's invisibility potion. Using this and the Zombie ability, Wario eventually finds himself in a room filled with invisible platforms, where he also can find the chest he is looking for.
Collecting the Red Bracelet does not affect the Music Box World in any way.
Reaction Abilities in this Level
Screenshots
The plain itself. 
The sky at daytime. 
The sky at nighttime. 
The cavern beneath the plain.
