How does the CAPTCHA system in the Stack Exchange platform work behind-the-scenes?

Specifically, after the system decides to display a challenge to the user, how is the result URL stored? Furthermore, in instances when a CAPTCHA is displayed during a form submission: how and where is the form data stored?

Edit: We already know that it uses ReCAPTCHA (based on many architecture blogposts/answers) - my question is about how the platform is structured around ReCAPTCHA; e.g. where the form data is stored.

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    Good try, Robot – Pekka Oct 2 '11 at 0:18
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    @Pekka: lol :D full disclosure: I'm making an ASP.NET MVC website where I'm trying to implement a similar system. – Maxim Zaslavsky Oct 2 '11 at 0:19

It uses reCAPTCHA.

As for the form data, I assume it is stored in <input type="hidden"> fields.

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ReCaptcha provides the actual captcha services. There is a Codeplex project for MVC ReCaptcha in C#. I haven't personally used it, but it looks to have good documentation and has good reviews. It might be a bit outdated though.

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