We train the machine so well, and it's use so ubiquitous, that it can become invisible: Google is making CAPTCHAs invisible using "a combination of machine learning and advanced risk analysis that adapts to new and emerging threats," Ars Technica reports. Emphasis added.
"The old reCAPTCHA system was pretty easy -- just a simple "I'm not a robot" checkbox would get people through your sign-up page. The new version is even simpler, and it doesn't use a challenge or checkbox. It works invisibly in the background, somehow, to identify bots from humans. [...] When sites switch over to the invisible CAPTCHA system, most users won't see CAPTCHAs at all, not even the "I'm not a robot" checkbox. If you are flagged as "suspicious" by the system, then it will display the usual challenges.
reCAPTCHA was bought by Google in 2009 and was used to put unsuspecting website users to work for Google. Some CAPTCHA systems create arbitrary problems for users to solve, but older reCAPTCHA challenges actually used problems Google's computers needed to solve but couldn't. Google digitizes millions of books, but sometimes the OCR (optical character recognition) software can't recognize a word, so that word is sent into the reCAPTCHA system for solving by humans. If you've ever solved a reCAPTCHA that looks like a set of numbers, those were from Google's camera-covered Street View cars, which whizz down the streets and identify house numbers. If the OCR software couldn't figure out a house number, that number was made into a CAPTCHA for solving by humans. The grid of pictures that would ask you to "select all the cats" was used to train computer image recognition algorithms."