Secret one
Climb through the collapsed floor area

and head through the door

at the T junction, head straight

you end up in a burial room. On the far wall there is a movable stone block - it's a button - press it

Pressing that button has opened a secret door in the neighbouring area - but that door only stays open for a few sections, so you need to sprint back to the T junction, and thus time turn right

Which leads you into a large room, at the centre of which is a four-way crossing. Quickly head left here

and sprint through the secret door before it closes

and into the secret chamber behind it

To escape, you need to hit that switch on the wall, which opens the secret door again.
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