E.g. GitHub /repos/:owner/:repo/pulls/comments/:comment_id
But flat is better than nested, esp if globally unique IDs are used already (and they often are).
Also it is possible to embed a sub resource (or part of it).
Think a blog post.
/blogPost/:blogPostId
You can embed a blog object with the url and title so you can show the blogpost on a page with the name of the blog in one go.
If you need more details on the blog you can request /blogs/:blogId
E.g. GitHub /repos/:owner/:repo/pulls/comments/:comment_id
But flat is better than nested, esp if globally unique IDs are used already (and they often are).