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This is kind of like a sidescrolling level in that its near impossible to go back, but the fact is it is a regular level with platforms floating on lava. At the start of the level, there are two flying ? blocks and one stationary one. The purpose of the first two (containing coins) is to boost you up to the last one, which contains a 1up mushroom. This is a rather dangerous gamble for inexperienced Mario gamers, for if you don't collect the 1up mushroom, you will be trapped on the blocks. "Up a creek without a paddle", as it were. Continue up the lava river on the skull platforms, and you should do fine.