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. Google doesn’t go into much detail on how it works, only saying that the system uses « a combination of machine learning and advanced risk analysis that adapts to new and emerging threats. » More detailed information on how the system

Source : Google’s reCAPTCHA turns “invisible,” will separate bots from people without challenges | Ars Technica