An option to be notified of responses to all comments under a a specific one

Sometimes I'll make or read a comment and want to stay abreast of all the conversation beneath it, including replies to replies and so on. I can obviously manually switch on notifications for each reply as they come in, but an option to do that automatically would be great.

Originally I was going to suggest it just for comments the user posts, but actually being able to switch it on for any comment, in the same way we can for single level notifications now, would make tracking some discussions a lot easier.