From the Super Mario Wiki
Skull Raft is a raft made out of bones. It debuted in the game Super Mario World. They are used to help Mario and Luigi to cross the lava river. They are only seen in a couple levels where there are levels filled with lava rivers. However, sometimes, Blarggs will jump out and try to attack Mario or Luigi, so they had to jump to avoid them, then land back on the Skull Raft.
The Skull Raft in Super Mario World
In the Super Mario World television series, a Skull Raft appeared in the episode, "Mama Luigi". Here, it shows that the Skull Raft is made of both wood and skulls, not just skulls like in the video game. During Luigi's story, Luigi got attacked by a Fire Sumo and fell into the Lava Pits. However, he landed on a Skull Raft, which saved his life. Unfortunately, he was heading for a lava waterfall, and Luigi and the Skull Raft fell down the waterfall, with Luigi landing on solid ground, where he found Yoshi.
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