Where are all of the secrets in the Hangar - E1M1?

I have played through the Hangar level of the first episode of DOOM now several times, but have not yet been able to complete the level with 100% in Kills, Items and Secrets. I am getting outside to pick up the blue power armour, and running back through to ensure that I'm not missing any items and kills;

I am going to suspect that the missing items are in the same places as the missing secrets, so where are all of the secrets in the Hangar - E1M1?