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All answers having "Bog" as clue:

3 Character Answers:

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FEN



4 Character Answers:

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MIRE

MOOR

OOZE

PEAT

QUAG

WASH



5 Character Answers:

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MARSH

SWAMP



6 Character Answers:

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MORASS

MUSKEG



7 Character Answers:

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WETLAND



8 Character Answers:

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QUAGMIRE