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It's not viewable on the map; it can't be detected with Batman's detective mode, and you'll need to use three explosive gels to crack open the wall, where normally you would only use one.
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These quickly plummet back to the ground destroying anything nearby.
For starters, Willy's pet toad is called Horny.
While developers nowadays make games that any player can beat to 100% completion provided they're patient enough, that hasn't always been the case.