Quake 1: All Secrets

Episode 1: The Doomed Dimension

Map e1m1: The Slipgate Complex (6 secrets)

Fresh off the slipgate, turn right and jump on the ledge with the flashing light. Shoot the wall with the red markings to find shells behind.
Take the lift down and clear the bridge. Jump in the water and get into the cave. Get the super health pack and continue along the wall, a lift will bring you up to the blue armour behind secret #1.
Take the round trip again - enter the door across the bridge and turn right. On the column is an earth picture on a monitor. Stand directly across it and shoot it, the floor lifts you to the ledge. Shoot the next monitor, and a secret door opens to Quad Damage.
To the left inside the door is a big room. Go up the stairs to the edge of an acid pool. Touch the button and a plate covers the acid. Halfway across, inside a hole in the left wall, is a red target. Shoot it, and a secret door opens to a double-barrelled shotgun.
By the way, across the acid and to the right, just before the slope is a different-textured bit of wall. That's the back door to secret #3.
This one will test your jumping abilities. Halfway down the slope, jump on the handrail. Then on to the lamp and the switch plate. Finally a running jump, aiming for the lowest step on the side of the wall. Hop up to the room to get another super health pack.
Go to the end of the slope under secret #5. Grab the biosuit from behind the pillar and jump into the acid. Swim under the door, and follow the tunnel to the circular opening. Above it is some health, a yellow armour and a slipgate.
Either go outside and jump in the water - pick up the Big Health kit, shoot the wall behind it and you are at the exit slipgate. Or walk around again, shoot the wall to the right, as you come up the slope in front of the slipgate, then you can pick up the Big Health kit dry-footed.

Map e1m2: Castle of the Damned (3 secrets)

Past the shooters, in the room with the yellow armour, is a secret door. Jump down in the water and walk towards the wall. Door opens all by itself to a health pack and shells. Use the slipgate inside.
After using the slipgate, you are on a ledge with blue armour. Jump in the water again and swim under the central bridge. Opposite the big red mark is the entry to a flooded room - in the far left corner is a ramp to some health packs and shotgun shells. Take the exit, up the stairs to the left, then in the water and up the ramp.
Exiting from secret #2, you will have the silver key door on your right, and some steps up to the left. Go up the stairs. On one side of the middle column a brick sticks out. Push it, and a door opens, revealing Quad Damage.

Map e1m3: The Necropolis (3 secrets)

Just down the stairs from the entry hall is this small bridge. Jump down. A bit of the wall has a different texture. Shoot or axe it, and get the health pack and rockets. A lift will bring you to the barricaded blue armour room.
When you go for the gold key, go to the back wall and dive to get a Ring of Shadows.
Shoot the two jerks up on the platforms. This draws the bars in front of the yellow armour. Shoot the wall behind the yellow armour and you can teleport up to the platforms for rockets and whatever the creatures dropped.

Map e1m4: The Grisly Grotto (3 secrets)

Go inside the first door and shoot the two switches inside, under the roof. This lowers the platform, crushing an enemy, and yields a yellow armour.
In the water-filled cave, a bit to the right and a few feet down is a small hole. Rockets inside.
Once you press the five floor switches in the Shooter room, two doors open. Drop down both of these, and press the wall switches. Don't forget the goodies. Quake tells you that "A secret cave has opened...." Finish the level, but don't exit - get back into the water-filled cave.
A big hole has opened underwater, roughly opposite secret #2. Here is a grenade launcher and a slipgate to a secret level: Ziggurat Vertigo.

Map e1m8: Ziggurat Vertigo (2 secrets)

Grab the Pentagram of Protection in front and walk into the lava. Enter the short tunnel under the walkway, opposite the pyramid, to get Quad Damage.
Enter the silver key door. To the right is the exit, to the left is the secret. Shoot the misaligned bit of wall for nails.

Map e1m5: Gloom Keep (5 secrets)

Entering the level dive off the bridge. To the right, near the bottom is the entry to a cave. Super health pack and shotgun shells inside.
Climbing out of the water after secret #1, you stand next to the back door. Enter. At the top of the big stairs, jump over to the L-shaped wall, from there to the sniper stand and yellow armour.
Entering the front door, go left on the Y-shaped platform, past the traps, and get the silver key. Sneak up the right side of the Y-platform. In the dark room with the single torch on a column, head-butt the torch. A secret door opens behind the column, revealing yellow armour.
Carry on up the stairs, to the slipgate room. If you enter the front of the slipgate, you will be sent to the gold key. Squeeze in the back of the slipgate, and you'll be sent to Quad Damage instead. Snatch it and run back into the slipgate.
When you land in front of the exit slipgate, right behind you is a misaligned bit of wall. Shoot or axe it. There is a slipgate for getting back up in case you get locked in, and blue armour.

Map e1m6: The Door to Chton (4 secrets)

You trigger a door by stepping on a floor switch. Opposite the switch, high up, is a target. Shoot it. It opens a door to a lift - ride it up. Jump down on the ledge and collect the Quad Damage.
Deep down, a squasher attempts to make you part of the wall decoration. Get behind it. Here is the slipgate to a super nail gun. From there you can also jump down to the right to a super health pack.
After securing access to the gold key, the red mark glows. Shoot it. The stairs fold down. Run down to get the rockets. Now, STAY!
...When the stairs rise in front of you, a slipgate is revealed. Enter the slipgate to appear opposite a super health pack just above.