Mud

 is a mixture of strata 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 results 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 reappears 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 DS
In Mario Kart DS, mud appears throughout Wario Stadium and in Delfino Square's shortcut prior to the bridge.

Mario Kart Wii
In Mario Kart Wii, mud reappears in Waluigi Stadium and Delfino Square. The cave section of has mud added to the off-road portions. The final area behind the Finish Line at Toad's Factory also features mud.

Mario Kart 8 / Mario Kart 8 Deluxe
In Mario Kart 8 and its Nintendo Switch port, mud appears on and Excitebike Arena.

Mario Kart Tour
Mud reappears in the returning courses SNES Donut Plains 1, SNES Choco Island 1, SNES Donut Plains 2, SNES Choco Island 2, SNES Donut Plains 3, GBA Riverside Park, GBA Sunset Wilds and DS Luigi's Mansion in Mario Kart Tour. Mud also appears in RMX Choco Island 1, RMX Choco Island 2 and RMX Donut Plains 1.

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 functions the same way as its predecessor. If Rock Mario rolls over mud, he does not slow down. It is found on the in Sky Station Galaxy and the  in Boulder Bowl Galaxy.

Dr. Mario World
In Dr. Mario World, mud is used to cover Muddy Coins. The mud on it can be removed by hitting the coins. Once it is cleaned, the player can receive the coin.