It sounds crazy, but "unravel" is a synonym of "ravel." "Ravel" is from the Dutch word "rafelen," which describes a thread unweaving from a fabric or spool and becoming tangled. English speakers understood the Dutch word two ways. From one point of view, it suggested unweaving, which lead to the sense of "ravel" that means "to disentangle." From another point of view, it suggested becoming tangled, which lead to the sense of "ravel" meaning "to entangle." People took the "to entangle" meaning of "ravel" and logically created "unravel" to mean "to disentangle." As a result, "ravel" has contradictory meanings, and "ravel" and "unravel" are synonymous.