The first hint to crack the puzzle "Bumpy, not flat, e.g. an __ surface" is:

It is a word which contains 6 letters

The second hint to crack the puzzle "Bumpy, not flat, e.g. an __ surface" is:

It starts with letter u

The third hint to crack the puzzle "Bumpy, not flat, e.g. an __ surface" is:

It ends with letter n

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The answer for the puzzle "Bumpy, not flat, e.g. an __ surface" is: