While the level is loading, when it starts, instantly move backwards to find a "generous yard". There's some ammo and various items there. Picking up the item behind the obelisk will cause the obelisk to collapse, and spawn some Lava Golems.
In the first room, there are 2 switches behind 2 columns, one on the left and one on the right. Press both switches to lower a platform next to the door where you entered from, which will then lift up again, transporting you to a secret area.
In the Ankh pool, dive in, and you'll notice there's a differently colored part on the floor of the pool, with some cannonballs above it. Touch it to reveal a secret room.
On the right side of the Ankh Pool, on the wall, is a differently colored part. Touch it to reveal a Croteam Papyrus and a Super Armor.
After swimming through the pool into the temple, turn around. Go to the left and right corners are some switches, press both of them to open a secret door near the obelisk, in the middle of the Ankh pool. The door leads to a room with a blob-like thing, a Super Health and an Ammo Pack.
Before the Marsh Hopper fight, in the room with the two pools, dive in the pool on the left, and touch the differently colored wall to reveal a path to a spherical room with abnormal gravity (in Serious Sam HD, this room has normal gravity).
In the last area, there is a differently colored small spot on the wall on the left. Walk through it to find a secret Ammo Pack.
Eventually, in the last area, it will say that the "Secret Health has been enabled" at some point (when the Werebull and Major Bio-mechanoid fight occurs). It is opposite of #7, pick it up to register the secret.
This level is split up into "Karnak" and "Ankh Pool" in Serious Sam: Xbox, where Karnak takes place in the day, and Ankh Pool in the night. Ankh Pool starts a bit before the area with the baboon statue of the god Thoth, followed by the Ankh Pool itself. Karnak uses Luxor's music, while Ankh Pool uses Karnak's music.
NETRICSA incorrectly identifies the god on the doors above the spikes as Thoth. It actually depicts Ra. This is because, in earlier versions of the game, Thoth actually appears on the doors but was later replaced by Ra and NETRICSA's analysis was not updated to match the change.