http://www.learnersdictionary.com/definition/plumb
3 plumb /?pl?m/ verb
plumbs; plumbed; plumbing
Learner's definition of PLUMB
[+ object]
1
a literary : to examine (something) in a careful and complete way in order to understand it
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The book plumbs the complexities of human relationships.
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a scientist who spent her life plumbing the minds of criminals
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The play plumbs the depths of human nature.
b : to experience or reach (something)
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someone who has plumbed the depths of loss/pain [=who has experienced a lot of loss/pain]
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The company's stocks plumbed new depths [=reached their lowest value] this week.