From the Super Mario Wiki
Urchin Underpass is a battle course that appears as one of the new maps in Mario Kart 8 Deluxe. The map is based on the main Turf War map from Splatoon and contains elements from that game, such as several green and purple ink splotches scattered through out the course, but they do not slow down the vehicles. There are also Mario Kart elements like Dash Panels. There are various paths that all loop around to the main area and several bridges. The music is a variant of the "Splattack" theme played during Turf War matches. When there is one minute left in a match, the music will change to a variant of "Now or Never", which also played at the last minute in Turf War matches.
Like with Waluigi Pinball, Hyrule Circuit, and Animal Crossing, the item roulette sound effect is a unique one to this course. In this case, it is the Stat Slot sound. In Coin Runners and Renegade Roundup, Splatoon's currency is used in place of Coins.
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