Serious Sam HD Demo Secrets

So I finally found all nine of them.  I know I'm a few years late to the party, but whatever.  Here's how to get each one.

1. When the level is done loading, but before you click Continue, hold S and keep holding it after you click Continue.  You'll back up through the door that closes at the beginning of the level.  You'll get the message "Generous Yard has been found!".  Run around and collect the weapons and ammo.  There are enemies in here.
2. In the large hall with all the pillars, one of them on each side will have a button.  Press both buttons, then get on the platform next to where you entered the room.  Explore the ledge to get the secret.
3. On difficulties under Serious, shoot the enemy in the middle of the room that has the rocket launcher on the far side, with your pistols, before entering the room.  On Serious, be in the room with the door closed and kill that same enemy without the rocket launcher.
4. In the area with the large pool of water, jump in the water right away.  Look at the bottom, you'll see some shotgun shells.  Go down to the bottom near the shotgun shells and the door to a secret room full of ammo and armor will open.
5. Immediately after leaving that, go towards the monument.  Turn right, and swim down to the middle of the far wall.  There will be a discolored portion of the wall (may be difficult to see).  Touch it and a door will open.  You'll get a message from Croteam (the devs), and an armor pickup.
6. After that, go through the underwater passage into the next area.  On the walls on either side of the door that won't open are two buttons, press them.  Go back through the underwater passage and over to the monument, which has now moved to uncover a secret underwater passage.  Swim down to get a large heart and an ammo pack.
7. In the room with all the ammo just before Netricsa says "Be careful, I smell a lot of bastards.", one of the pools has a discolored wall that will open when you touch it.  Swim down to find an air-filled chamber with a bunch of armor powerups.  Enemies will spawn as you pick these up.
8. In the final area where you're being rushed, partway through the fight you will see the message "Secret Health has been activated!"  Look at the middle of the side walls, you'll see the heart in one of them.  Go get.
9. On the opposite side of the area from that, there's a discolored wall that you can walk through to get Secret Ammo.

Now you know.