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 7
Some mud appears at the starting section of Maka Wuhu in Mario Kart 7.

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
In Mario Kart Tour, the mud found in the returning courses SNES Choco Island 1, SNES Choco Island 2, GBA Sunset Wilds, and DS Luigi's Mansion no longer slows drivers down, instead fulfilling an entirely aesthetic purpose. This type of mud also appears in RMX Choco Island 1 and RMX Choco Island 2.

Patches of thicker-looking mud appear in various other courses, such as SNES Donut Plains 1, SNES Donut Plains 2, SNES Donut Plains 3, GBA Riverside Park, and RMX Donut Plains 1. They typically cover areas closer to the inner edge of a wide curve. They slow drivers down to an extent, much like off-road sections do, but are shown as being part of the drivable track in minimaps. Drivers can also perform Mini-Turbo actions while on this type of mud, albeit without getting speed boosts.

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.