404 Page not found pages show up when the user ends up on a route that does not exist. This might be because they included a typo as they manually entered the URL in their browser. It can also happen when they follow a broken link from your site or even an external one. Adding a custom 404 error page to your Astro project is quite straightforward, though. First create a `src/pages/404.astro` file in your project. You can edit this and include layouts, just like you would any other Astro page. You might consider capturing the URL entered, as this can also help you in updating broken links. Anyway, Astro will build this page to /404.html. Importantly, most static site hosts will look there for an error page to render whenever needed.