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Citizens of an underwater city along the Ring of Fire are trying to protect a 50kg object. The object is housed in an underground cavern near the city. How can motion be passively detected in or near the cave?

Due to the location of the city, P-waves caused by earthquakes are common.

I also wonder if an infrared motion detector, specifically, could work due to the amount of organic stuff, like plankton, floating in the water.

What could I use to passively detect motion underwater?

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    $\begingroup$ I have edited your question to avoid closure due to its (frankly, unnecessary) dependency on 3rd-party/commercial worlds. We do not answer questions about 3rd-party or commercial worlds. Please note that your question may yet be closed due to lack of details. How far underwater? What detection range are you expecting? Do you need to detect in two or three dimensions, and across how many degrees of arc? Etc. $\endgroup$
    – JBH
    Commented Jan 1 at 6:58
  • $\begingroup$ BTW, infrared detection has nothing to do with "organic stuff" being in the way. One can detect heat through vegetation and walls. Underwater thermal vents, on the other hand, would make infrared detectors next to useless... depending on how the detection software was programmed. Advanced detection systems (infrared detectors, cameras, etc.) can be programmed to exclude zones of interference. An example is security cameras, which can be programmed to ignore portions of the image due to sunlight overwhelming that one spot (e.g. a window) and causing false positives. $\endgroup$
    – JBH
    Commented Jan 1 at 7:00
  • $\begingroup$ Are these potential thieves warm-blooded? As We've learned from some film or other, a wet-suite can protect the wearer from detection by thermal sensors. $\endgroup$ Commented Jan 1 at 12:57
  • $\begingroup$ Just use a mechanical mock up if an inner ear with the ear drum removed. $\endgroup$
    – Pelinore
    Commented Jan 1 at 15:14

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Pressure waves are easy to detect underwater (source: fishes use the lateral line to detect motion caused pressure waves even in muddy waters). Just install rings of detectors at known distances and locations, and have their signals elaborated by a central unit.

Knowing magnitude of the pressure waves and their propagation velocity it will be easy to triangulate the location of the source and tell the difference between a distant earthquake and the notorious thief Lufin the 33rd moving around the treasure room.

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