To find the secret exit in Doom 64 map 12, ALTAR OF PAIN,
which leads to map 30, THE LAIR:

As you pick up the yellow key, take a look at the Automap.
Nearby is a small platform rising up from the water, close
to the edge of the map.
In the Automap view, this platform will look somewhat like
a mushroom, or a short and thick arrow shape.
This "arrow" points towards a raised opening hidden in a
false section of the wall at the edge of the map.
Get on to that "arrow" platform, and Run across it, towards
the section of wall it points to, and you'll go through the
hidden opening.
Follow the short passage around to the secret exit.

Hope this helps. :)                                                

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