My Bloody Valentine

Inside a mine shaft, a female miner takes off her gear while another miner remains geared. When the woman performs a strip tease and fondles with the miner's breathing tube, she is brutally killed by an embedded mining pick.
Mayor Hanniger (Larry Reynolds) of mining town Valentine Bluffs reinstates the traditional Valentine's Day dance which has been suspended for twenty years. The dances stopped after an accident that occurred when two supervisors left several miners in the mines to attend the dance. Harry Warden, the only survivor who resorted to cannibalism, murdered the supervisors who left the mine and vows further attacks if the dance resume. As the accident was forgotten and Warden was placed into an asylum, the dance resumes. Many of the town's younger residents are excited about the dance. Of this group Sarah (Lori Hallier), Axel (Neil Affleck), and the mayor's son T.J. (Paul Kelman) are involved in a tense love triangle.
Mayor Hanniger and the town's police chief Jake Newby (Don Francks) receive an anonymous box of Valentine chocolates containing a human heart and a note warning that murders will begin if the dance proceeds. That evening, resident Mabel (Patricia Hamilton) is murdered by a mining-geared killer with her heart removed. When Newby discovers this the next morning, he publicly reports that she died of a heart attack to prevent a panic. Newby contacts the mental institution where Harry Warden was incarcerated, but they have no record of him. Hanniger and Newby cancel the dance and order the hall locked, however the town's youngsters decide to hold their own party at the mine head. After a bartender named Happy (Jack Van Evera) is angered by the youngsters, he tries to set up a fake miner dummy to scare them, only to be brutally killed by the real miner.
Soon, the miner proceeds to kill many of the youngsters, including some down at the mine head. After Axel and T.J. have an angry confrontation, a shaken Sarah and five others decide to travel down into the mine as a diversion. Newby, after getting information from Gretchen (Gina Dick) about the murders, begins to rush into the mines with police support to rescue the group. Down in the mines, T.J and Axel lead the remaining party to safety while in pursuit by the miner. Many of the group falls victim to the miner as he chases after the remaining T.J and Sarah until leading to a small alcove. During the fight, the miner is revealed to be Axel, who faked his death earlier. In a flashback, a young Axel witnessed his father (being one of the supervisors) being murdered by Harry Warden and was since traumatized by the event. Soon, the tunnel begins to collapse from the battle and traps Axel inside while Newby and the police arrive to rescue T.J. and Sarah and it is revealed that Harry Warden had died five years earlier. When Sarah is emotionally prompted by Axel's screams, she sees Axel freeing himself from the fallen debris by amputating his trapped arm. As Axel begins to run into the darker mines, he rants about returning to kill everyone and mumbles for Sarah to be his bloody valentine. The film concludes with a maniacal laughter being heard and a ballad for Harry Warden begins to play over the credits.

Question: What did the box of Valentine chocolates contains?
Answer: a human heart.