The Tunnel of Lost Love, known as the Curséd Cave of Broken Loves in the manga, is a genuinely haunted Kimodameshi, the Japanese equivalent of a haunted house, established in a deep, long cavern. As the name implies, the tunnel seemingly exists for the purpose of shattering relationships; the many ghosts in the cavern not only materialize for the purpose of frightening one member of a couple (only male-female pairs are allowed access to the cave) while failing to appear before the other, thus causing the pair to bicker and argue about whether it truly is haunted, if the couple somehow makes it to the final exit without looking like they are arguing, the ghosts materialize en masse to break them up by force. Even then, there are other obstacles to get past.
After making it through the half of the cave where the first hauntings take place, the couples must row boats across a cold river to the other side - but the boats are deliberately maintained in shoddy condition; they can't take the weight of more than one person without collapsing, prompting the typical couple to fight over who will be forced to swim.
On the other side, the bathrooms are seperated by sex; the women's bath is a luxuriant, well-stocked, fully modern affair that encourages them to linger and enjoy themselves. The men's bath, on the other hand, looks like a scene from a depiction of the Buddhist Hells; it consists of a giant iron pot being heaten over a fire, and most men aren't willing to get in. This means that by the time the women come out, the men are typically freezing cold and ticked off, which usually leads to arguments.
And even if it doesn't, the next step is the chasm of partings, where the ghosts make their literal final assault.
The Tunnel of Lost Love actively boasts about its skill at seperating once-loving couples and claims that those who are broken up there will never be able to become a couple again.
In the manga, Ranma and Akane are lured here by Ukyo as part of a plan to break them up, with Ukyo being forced to team up with Ryoga to get in and make sure their squabbling doesn't ruin things before the place has truly broken up. However, their ability to focus on their mutual goal (as well as their squabbling) means that not only do Ranma and Akane mistake them as having become a couple, due to how similar to Ranma and Akane they appear to be, but the actual ghosts attack Ryoga and Ukyo and claim that they look much more like a couple than their true targets do. They are shown afterwards blaming each other viciously over how things went wrong, and the implication is that they are another "victim" of the anti-relationship curse.
In the anime, the plot is masterminded by Ukyo and Shampoo, and the whole crew comes along for the ride, though the fundamental point of Ukyo and Ryoga trying to work together to break up Akane and Ranma remains the same. The OAV ends with them just having fought their way free, finding that their plan failed as Ranma and Akane have just gone back to being how they were before they started arguing, and collapsing in disappointment and exhaustion.