@Avi and @Bastien_Siebman, it actually is possible, but I have to lead off with a “we discourage it” for a pretty good reason - it’s possible to cause an Asana API stability issue with this. It makes for a somewhat dangerous situation, especially if you’re building an integration for many users, because the amount of data that comes back depends on the structure of the subtasks. Your integration might be fine, then bam, a pathological case that causes us to ban your app for that user 
I’ve started some dialogues with the API team about whether we can mitigate this to some degree, but in the future, the mitigation will probably look quite different than what we allow at present - it will probably be either something meant to traverse nested graphs like http://graphql.org/ or we just remove the method by which you can do this today, or both, so how to do it right now is likely to change in the future.
That being said, it can be pretty handy if you absolutely know that you’ll never ask for a pathological subtask tree, so rather than go through how it can be done here (in case others stumble on this without reading the caveats above ^ ) I’ll DM you with how to do it.
Cheers!