There are many techniques which as considered Black Hat SEO techniques. Some of them which are commonly used are :
Keyword stuffing: Packing long lists of keywords and nothing else onto your site will get you penalized eventually by search engines.
Invisible text: This is putting lists of keywords in white text on a white background in hopes of attracting more search engine spiders.
Doorway Pages: A doorway page is basically a “fake” page that the user will never see. It is purely for search engine spiders, and attempts to trick them into indexing the site higher.

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