Beauty and the Beast (TV series)

A 1987-1989 TV series focusing on both mysteries and a fantasy romance between Catherine Chandler (played by Linda Hamilton) and Vincent (played by Ron Perlman).

The show arguably influenced later series Dark Angel (TV series), which also featured an unresolved sexual tension between the female protagonist and an underground (not literally) leader; and which in its second season developed a literal underground of a series of mutant human-animal hybrids, including a character (Joshua) which somewhat resembled Vincent. The fantasy/mystery/romance mixture was also successful in The X-Files.