Oh wow, they just disabled it while I was reading some comments. It's no longer working, I'm now getting redirected to nsa.gov
Edit: This seems to have been online since 2018, see https://web.archive.org/web/20181206224407/http://captcha.ns....
Edit: This seems to have been online since 2018, see https://web.archive.org/web/20181206224407/http://captcha.ns....
The legit explanation (given the domain name) is probably they wanted to use reCAPTCHA, but block all non-NSA hosts with a firewall or something.
This is not great, because the NSA expanded its attack surface to all of google.com.
The more conspiracy explanation is that this is actually a phishing page set up, and due to a misconfiguration it's exposed under captcha.nsa.gov, but Occam's Razor should apply here.