I'm using the term "shutter" to mean an obstruction that blocks a hole; a valve usually has a more complicated structure, but it does basically the same thing. The most common shutter, of course, is a door or lid; when closed, it can not only block access or vision, but it can also prevent the release of tension in spring triggers, as we've already seen. Another trigger can be used to open a shutter, releasing dangerous substances. Consider this: subterranean dwellers discover a large underground vent of deadly gas; they carve a channel or construct a pipe through rock to direct the gas to a particular area, then cover the vents with shutters, opened by another trigger. A similar concept is to use a shutter as a floodgate; opening the floodgate dumps water (or sand or boiling oil.)
Shutters can be left open by default instead, with a mechanism to close them when triggered. This is your basic portcullis trap: the gate is raised by default with a catch to keep it in position, with a trigger to release the catch, dropping the gate and preventing access.
Detecting shutters is usually pretty easy visually, except in darkness. Tapping on the wall should also reveal hollow areas behind shutters. If the shutter blocks release of a gas, liquid, or grainy substance, there will probably be some leakage, either visible or perhaps detectable as an odor. If the shutter is open, either adventurers will see the propped-open shutter, or there will be a hole in the threshold of the route the shutter would block. Disabling a shutter trigger usually involves a wedge to keep it open or a spike to keep it shut.
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