Piranha

Two teenagers find an abandoned military compound and skinny dip in a holding pool, but are attacked and killed by unseen creatures.

Sometime later, determined yet absent-minded skiptracer Maggie McKeown is dispatched to find the missing teenagers near Lost River Lake and hires a surly backwoods drunkard, Paul Grogan, to guide her. They discover the compound and find bizarre jarred specimens and evidence of an occupant inside. Locating a drainage switch for the nearby pool, Maggie empties it to search the bottom, but a haggard man enters and tries to stop her until Paul subdues him. The pair finds a skeleton in the pool's filtration trap and learn it was filled with saltwater. The man awakens and steals their jeep, but crashes due to being disoriented and is taken to Paul's home. The next day, as the three travel downriver, the stranger tells the pair that the pool was filled with a school of piranha and Maggie released them into the river. She and Paul are skeptical until they come across the corpse of Paul's friend Jack.