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From The Collaborative International Dictionary of English v.0.48:
underlying \un`der*ly"ing\, a. 1. Lying under or beneath; as, the underlying strata of a locality. [1913 Webster] 2. Hence: Fundamental; basic; as, underlying principles; underlying causes. [1913 Webster +PJC] 3. Implicit; not immediately obvious; requiring careful scrutiny to discover; as, the underlying sarcasm in her seemingly innocuous remark.. [PJC]