Lots of work going on to learn how to break captchas today.

Columbia University is trying to figure out how to defeat vidoop (http://www.cs.columbia.edu/~mmerler/...l%20Report.pdf)

WIRED had an article on breaking captcha as far back as 2013 https://www.wired.com/2013/10/captcha-busted/

GitHub even has a library of scripts to defeat captcha https://github.com/imsky/captcha-breaking-library

We had been hit by so many bots back in 2011 that I actually removed contact forms from some of my sites. It seemed to "attract" the bots which were unbothered by captcha images. I'm hoping that using images of text or numbers with constant motion in the background or changing backgrounds I'll be able to continue to keep the bots out of my sites' contact forms.