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Great ___ Reef (7 letter answer)
The answer BARRIER is rarely seen, appearing only about once every 2100 puzzles.
BARRIER as a noun:
1. a structure or object that impedes free movement
2. (barrier, roadblock) = any condition that makes it difficult to make progress or to achieve an objective; "intolerance is a barrier to understanding"
3. anything serving to maintain separation by obstructing vision or access