The first secret (sector #225) is a small place behind the blue key, open it inside the small room with two demons (linedef #1137).
The second and third secrets (sectors #199 and #259) are near the red room with lava, lost souls and the first baron, behind the left wall of the little room full of monsters.
The fourth secret (sector #251) is on the right after the yellow key door, on the left in the spectre cache.
The fifth secret (sector #127) is behind the red skull keycard, go here from the left, where the baron is severed.
The sixth secret (sector #264) is the left cacodemon cache.
The seventh secret is in the wood room with stairs, on the right (get the soulsphere).
The last secret (sector #151) is in this wooden section, where you see a sergeant on your left. You must hit the switch (linedef #740) on your right just after you pop out the tunnel that goes to the marble section (do not miss it when you pop out).