Desperately Diving for Pearls of Great Price

You would be shocked to learn how fast you can make it to the bottom of a lake with a cinder block tied to your leg. I know this rate, by my own empirical investigation, to be thirty feet per second, give or take. If you’re in the mind to disconfirm this experimentally determined value, I will tell you how to repeat this experiment for yourself. First, convince yourself there are freshwater pearls in the mussels that live in your grandfather’s lake…

This is an example of the fresh water pearl mussel.  I never found any. If you aren't careful, you might, too.

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(Image obtained from http://upload.wikimedia.org/wikipedia/commons/e/ed/Margaritifera_margaritifera-buiten.jpg)