Oh, Cursed Tunnel of Lost Love! Let My Love Be Forever

Aa! Noroi no Harendō! Waga Ai wa Eien ni (ああ呪いの破恋洞！　我が愛は永遠に) is the ninth OVA of the Ranma ½ anime.

Shampoo and Ukyo arrange to travel to a cave that is legendary for breaking up couples. They hope they will cause Ranma and Akane to break-up and Shampoo hopes to ditch Mousse the same way. When Ryoga arrives Ukyo tells him what's happening and he happily joins in to help wreak Akane and Ranma's relationship. Ranma and Akane become convinced that Ryoga and Ukyo are now a couple and wish them the best, and the ghosts haunting the cave wind up attempting to break up Ukyo and Ryoga instead of Ranma and Akane.

Trivia

 * The anime version includes a much wider cast than the manga counterpart. In the manga Ukyo, Ryoga, Ranma and Akane are the only ones to enter the tunnel.