(Dedicated to Dan)
If you are thrown in at the deep end, you are dropped straight into a situation without any warning or preparation.
This can particularly happen when starting a new job.
The origin comes from the swimming pool which has a deep end and a shallow end.
People used to say that the best way to teach someone to swim, is to throw them in at the deep end. (Not something I would advise)
Here's someone who is definitely in at the deep end!