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BioShock reference
Level 9-7 from the "Dark Hollow" world is clearly inspired by BioShock. There is a "Giant" wearing a "Diving Helmet", resembling a Big Daddy, guarding a "Girl", called "Smaller Sister" in the hint (which is a reference to BioShock's "Little Sister"). Also, the level is designed as an underwater city, much like "Rapture" in BioShock.