Luis von Ahn, who worked on the team that created CAPTCHAs and became the face of the technology, armed himself with a theory of human computation and developed reCAPTCHA. reCAPTCHA is a free CAPTCHA service that contributes to the digitization of books, old newspapers, and et al. It mostly concerns itself with dead technologies and outdated media. According to some measurements, CAPTCHAs were being solved at a rate of 100 million a day. reCAPTCHA, rather than merely generating a randomized, nonsensical puzzle like its predecessor, imports unidentifiable text that optical character recognition software stumbles over during archival digitization into new CAPTCHA puzzles. reCAPTCHA was bought by Google (who else?) for its Google Books project and subsequently re-sold to websites across the web. Though it's marketed as a free service, remember TANSTAAFL. It became another one of the many tools that Google can offer. There was money to be made yet!
reCAPTCHA, in effort to process what are actually defined as un-processable words, must hedge the original bet of CAPTCHAs.  The user in reCAPTCHA is presented with two words, one of which the computer knows and the other it does not. If the user correctly inputs the first control word, it is assumed the user is human and has correctly solved the second, unknown word. Predictably, none other than crowd-sourcing solves the essential problem of (the) unknowability (of the second word). At least three users must agree on the second word before it is submitted as a viable answer.