Secret areas in GBA Doom II level 16, Industrial Zone: B:

There are 9 Secret-trigger lines on this map, divided among
8 areas.  The maximum Secrets score for this map is 75%.

Secret 1: When you first appear on the Industrial Zone: B
map, you'll be facing west, looking down a hallway towards
a square demon-faced panel that's high up in the far wall.
Just ahead of you near your starting point is an opening to
a stairway on your left.
Shoot the demon-faced panel to open a hidden chamber at the
bottom of the staircase.  Step inside to get credit for
discovering a Secret.

Go up the steps, and as you get to the top, turn left to
face a brown metallic wall panel. This is an elevator;
Activate it, get on and ride up.
At the top, turn left (east), and you'll be facing a torch.
On your left is a stone wall.  Follow this wall on your
left past the torch and around the corner (south a few
steps, then east, then north) until you get to a teleporter
pad between two more torches.
Step on the teleporter and you'll materialize on another
teleporter at one end of a short hallway.  At the other end
is a switch on the wall; Activate it and step back on the
teleporter.
You'll materialize back on the first teleporter.  On your
right is the stone wall.  Follow that wall back around the
corner to the elevator you rode on a minute ago.
Go back down the elevator.  At the bottom, turn east and go
back down the stairs.
At the bottom of the steps, walk around the corner on your
left, and walk west a few paces until you get to a narrow
bridge on your right.
Go across the bridge (north) and go left.  At the corner go
left again, then go up the steps (south) to a wooden door.
Open the door and grab the yellow key you'll find there.
Go back down the steps, and at the bottom look ahead and to
the left (north/northwest) for a teleporter in the corner.
Walk up and step on.
You'll materialize on another teleporter pad outdoors on
the grass.  You'll be facing northeast, and your back will
be towards a corner formed by tall stone walls.
Walk a few paces east, to where the grass ends at the edge
of a low river of crusted lava.
Face east towards the widest part of the lava river, and
you should see a small rectangular pedestal jutting up from
the lava below.  On top of the pedestal will be a radiation
suit, and just behind the suit is a switch mounted on a
small vertical slab.
Getting onto the pedestal and Activating that switch is the
next objective...
Turn left a little to face northeast, and you'll see tall,
greenish stone walls rising up from the lava below.  There
is another teleporter pad located down in the lava where
these stone walls come together at a corner.
Jump down to the crusted lava, and run onto the teleporter.
You'll materialize back near the top of the elevator you
rode a few minutes ago.  You'll be facing south, looking
towards where the checkered floor ends at a high dropoff.
From where you stand now you can run south off the dropoff
and land on the pedestal where the switch is.
This can be tricky... It's easy to run too far, or not far
enough, and land down in the crusted-lava river.
If you miss the pedestal or fall off, turn right/northwest
and follow the greenish stone walls, keeping them on your
right, until you get to that teleporter in the lava where
the walls form a corner; Then try again for the pedestal.
Once you land on the pedestal, Activate the switch.  This
opens a passagway at the far east end of the crusted-lava
river.  Jump down and run northeast and east to that end
of the river.

Secret 2: Enter the wide opening and follow the passage
around to a chamber where you'll see an Invulnerability
Artifact (a green sphere) on a wide, low platform in the
lava.  Step up onto wide platform to get credit for
discovering a Secret.

The Artifact itself rests upon a flashing stone rectangle.
Stepping onto or across this flashing rectangle will open
a secret-exit passage at a distant location on the map.
(That passage will open whether or not you pick up the
object, just so long as you step onto the flashing stone
rectangle where it rests.)
Step onto the flashing rectangle and pick up the object if
you wish; then run back out across the river, and run back
to the teleporter in the lava, and step on.

Again you'll materialize back by the top of the elevator,
and you'll be facing south, looking towards where the floor
ends at a high dropoff.
Walk a few paces south, to the end of the dropoff, and turn
right to face west.  In the distance you'll see where stone
battlements rise up from the grass below.
Run west off the dropoff, through the air above the lava to
land on the grass.  (If you fall down to the lava, run back
to the teleporter and try again.)
Continue northwest on the grass (keeping the river to your
right, battlements on your left), until you get to a
yellow-keyed door between two torches on your left.
Enter through the yellow-keyed door.  There's a radiation
suit in a little alcove on your right as you walk in.
In the floor is a crusted-lava stream.  Follow this stream
south, into a darkened tunnel.

Secret 3: On your left you'll notice another tunnel that
branches off to the side.  Step into this tunnel to get
credit for discovering a Secret.
Note: This tunnel curves around to a secret-exit teleporter
which will take you out of this level if you step on it.

Step back into the dark tunnel, and follow it clockwise and
around to its far end, and Activate the switch you'll find
there.
Go all the way back out of the dark tunnel, and just as you
emerge, look to your right for a stairway.

Secret 4: Start walking up the steps, and at the first
landing, where the stairs begin to turn left, search the
east wall for a secret panel, which will open to reveal an
inactive teleporter.  Step on to get credit for discovering
a Secret.

Continue to follow the winding path of stairs all the way
up as far as they go.
You'll come to a face-like switch on your right.  Activate
it to lower a small lift.  Get on and ride up.
Turn right and head east, around the top of the rampart,
until you get to another face-like switch.  Activate it to
remove a barrier that blocks access to the blue key, which
you'll acquire later.
Turn around and walk back along the path at the top of the
tower, continuing west, then north, until you come to
another switch.  This switch controls a tall, narrow lift
that is just to the west of your location, and its entrance
is located below, and just inside and to the southwest of
the yellow-keyed door.
Activate this switch, and quickly step off the tower to
drop down next to the yellow-keyed door.  Quickly go
through the door and around to your right, to step into the
small alcove and onto the lowered lift.
(If the lift raised again before you got on, simply go back
up the stairs inside the tower, and try again.)

Secret 5: Once you ride the lift up, step off onto a small
platform to get credit for discovering a Secret.

Go back down to the inside of the base of this tower.  Just
to the southwest of that tall, narrow lift, you should now
see an indentation in the wall.  Open this panel to reveal
the blue key.  Grab it, and then exit north through the
yellow-keyed door.
Follow the grass southeast, then east through a covered
passage where you'll need the yellow key to open some bars
blocking your progress.
Continue east to the other end of the passage, and open the
yellow-keyed door ahead.
Inside and just ahead you'll see a pair of blue torches.
Walk towards them, and a wooden barrier ahead will drop
down as you approach.
Step past the barrier, and walk north up the steps.  At the
first landing, where the winding stairway turns left,
you'll see a blue torch in the corner.  Just to the left of
that blue torch is a panel on the north wall that is
controlled by a tripwire located further up the stairway
and to the south.
Follow the wooden stairs up and around, and as you begin to
move south, a wooden barrier ahead will rise up, allowing
you to proceed south another few paces.
Just as you approach a second moving barrier to the south,
turn left to face east, take a step or two, and turn left
again to face north.
You'll see a short wooden tunnel in front of you.  Stand
inside the tunnel and face north.
Across a small gap is a blank wooden wall.  This is the
south face of a narrow chamber which contains a Megasphere.
Like the secret panel on the north wall of the stairway,
the panel blocking access to this chamber is controlled by
a tripwire on the stairway.
These secret areas can be tricky to access.  It requires
some timing and some careful control.
Go back down to the bottom of the wooden stairway, and
prepare to run up and around through the wooden tunnel.

Secrets 6 and 7: From the base of the stairway, Run up and
around, through the wooden tunnel, to jump into the
momentarily-opened chamber which contains the Megasphere.
Stepping across the edge of the chamber, and then into the
chamber itself, will give credit for 2 Secrets discovered.

Secret 8: From the base of the stairway, Run up and around,
through the wooden tunnel, back down into the stairway and
up to the momentarily-opened north wall.
Drop down into the narrow passage beyond to get credit for
discovering a Secret.

Head east, towards the flashing light at the end of the
passage, and turn left.  Activate the switch you find, and
a teleporter will momentarily open up a few paces to the
west.

Secret 9: Step onto the teleporter to materialize on a low
pedestal to the southwest, and get credit for discovering a
Secret.