>Nevertheless I tried Google and kept working through 15 (yes, fifteen) captchas in a row and Google still wouldn't let me through, and then I gave up. What the hell? Surely even after 1 captcha, Google should be amply convinced that I'm not a robot and let me did a single search.

Google probably identified you as computator prior to the capcha but that profile had no value for seemingly impossibly.

>Can anyone explain the purpose of Google putting up an apparently impossible barrier like that?

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