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yes, part of it could be dynamic sitemaps, however consider these factors: 1.  According to their Moz ranking, Yelp has a Domain Authority of 93/100 - that's a lot of domain authority. 2. They've got over 16 million pages indexed at Google. 3. Of these, about 5 million are search results 4. Google claims to be able to explore forms - their bots are claimed to be able to simulate human behavior in form usage. 5. With about ten million pages indexed that are not yelp search results pages, that's an awful lot of raw data for Google's bots to experiment with in trying out that search for.