After you obtain the mole's suit in the first stage, dig down and left as far as you can go and you will discover a little hole. No, this is not a pit that will kill you, it leads to a secret area with a lizard and a 1up.
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Put the lizard to sleep so you can ride him. Now go back to where you found the mole and jump over him so you can climb up the wall. Keep climbing as high as you can and you will find yet another 1up.
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After you get the hermit crab dig down as far as you can go. A secret area can be found if you tunnel under the section of the screen I have circled in black.
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The secret area contains an evil tree and a 1up. Evil tree? Oh no! Whatever will we do? Don't sweat it, your crab pincers will shred him to fine paper in no time. Now grab the 1-up and get back to finding those keys!
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