Your typical haunted house might make your heart race, your skin shiver, and your knees go weak — but all in all, they tend to be pretty predictable. You'll see a few cobwebs, hear a few screams, and get jumped at by a few people wearing cheesy rubber masks.

But for Halloween junkies who really want thrills, there's another option: haunted houses so boundary-pushing that you need to sign a waiver just to get in the door. They're called extreme haunted houses, and more of them are popping up across the United States every year. Each one is different, but they tend to include a few common characteristics: simulated assault and torture, and a safe word for anyone who can't handle it.

Why would anyone voluntarily subject themselves to so much unpleasantness? I reached out to Timothy Haskell, the creator of an extreme haunted house called "Nightmare: New York" — which has included interactive tableaus based on Ted Bundy, Lizzie Borden, and Jeffrey Dahmer, among other horrors — to learn more about the trend. "There is the old guard that has things just jump out at you," he said, "and then there's us."

"People love being scared because it gets the adrenaline going," he said. "We have had people throw up in the House of Fear; a ton of knee-jerk punch throwing; and many girls who try to make out with the bad guys — which always surprises me. But there is something about fear and sexuality that work in concert with each other."

So what are these extreme haunted houses like? At Ohio's Haunted Hoochie at Dead Acres, actors have been known to have full contact with guests, throwing attendees over their shoulders and spinning them around. Las Vegas' The Freakling Brothers: Gates of Hell specializes in phobias, subjecting guests to electric shocks, a fire-breathing clown that bites people's ears, and attackers knocking people down and dragging them into darkness. And that's just the regular experience; in the "uncensored" version, guests are forced to go alone, and there's a chance that they'll be subjected to light torture, including simulated drowning.

San Diego's McKamey Manor requires both an application process and a waiver. Unlike most of its rivals, there is no safety word. The tour, which is free, can take up to four hours to complete. In that time, guests have been tied and gagged, forced into coffins and freezers, and had their heads crammed into cages full of snakes. People have even been known to leave with scratches and bruises on their bodies.

The reigning champ is Blackout, which has versions in Los Angeles, Chicago, Miami, and New York, and has been rated on several lists as the most extreme haunted house in the world. Co-creator Josh Randall has one goal in mind: to create a psychological experience, where people can project a lot of their own fears and issues onto a relatively blank canvas. "Darkness, nudity, sensory deprivation, and other tactics along those lines strip the experience of false threats and can show something that, in our minds, is a little darker and more intense," he explained. "When the actors or the audience don't have something to hide behind, it creates a more visceral experience."

In the past, Blackout guests have experienced simulated rape scenarios; force-feeding of mysterious, disgusting things; suffocation; full-frontal nudity; and a walk through a darkened hallway of used condoms.

Of course, scare tactics this extreme have plenty of critics who charge that the houses' idea of "scary" has crossed (or will inevitably cross) into "dangerous." In some cases, extreme haunted houses have been slapped with lawsuits. But every year, there are plenty of people who can't get enough: horror movie aficionados, torture-porn junkies, and seemingly normal folks who just make it a habit to keep going back every year, wanting to experience the next thrilling step in the evolution of haunted houses. Even the veterans don't know what to expect; most houses, anticipating return customers, re-craft the experience ever year. And while they may go too far someday, the high prices and long lines are proof that wherever the line is, hardcore horror fans don't think it's been crossed yet.