Some exclusive versions use a one-time password (OTP) or CAPTCHA to ensure you are a human, not a bot trying to report the proxy.
Once the Google Site loads, you will not see a standard website. You will see a minimalistic interface—usually a textbox or a button that says "Launch Nebula Tunnel."
For any serious online activity, Nebula and other free proxies fall short of the reliability and security offered by professional proxy services. The table below shows a comparison: