Mud

is a mixture of a solid material and water. Mud is used multiple times in the Mario franchise as an obstacle, usually slowing the player down.

Super Mario Kart
Mud appears in Super Mario Kart, in both Choco Island 1 and Choco Island 2. In both courses, they appear as puddles throughout the track; driving through them will result in the player slowing down and mud being thrown behind the kart. In Choco Island 2 specifically, a large portion of the track is taken up by a mud puddle.

Mario Kart: Super Circuit
Mud reappear in Mario Kart: Super Circuit, reappearing in the same tracks with the same mechanics as before, though mud no longer is shown being thrown behind the kart and the mud has a lighter texture.

Mario Kart: Double Dash!
In Mario Kart: Double Dash!!, mud appears throughout Waluigi Stadium, composing most of the track's off-road areas.

Mario Kart Wii
In Mario Kart Wii, mud reappears alongside Waluigi Stadium.

Super Mario Galaxy
In Super Mario Galaxy, mud is found on the in Good Egg Galaxy. Various sections of mud are on this planet, and standing in it will result in Mario's movement speed and jump height being reduced.

Super Mario Galaxy 2
In Super Mario Galaxy 2, mud is found on the in Sky Station Galaxy. It functions the same way as its predecessor.