Arthur Levine

Special for USA TODAY

Imagine a large, outdoor water park that's stocked with nearly every conceivable slide and splashtastic attraction that the genre has to offer including a wave pool, a meandering lazy river, a water coaster, a funnel ride, a FlowRider wave-making machine for boogie boarding, a bowl ride, and much more. Now imagine cramming all of it inside a cavernous building.

That might begin to give you a sense of the gargantuan indoor water park at the Kalahari Resort in Pennsylvania's Pocono Mountains. When it first opened in 2015, the park was 100,000 square feet, which was quite large compared to similar facilities. An expansion that opened in March 2017 more than doubled the size of the indoor water park to 220,000 square feet, which the Kalahari says makes it the nation's largest. It's now considerably bigger than the average big box store — and many stories taller.

Really, the park is even bigger than advertised. All of the water slides start and end indoors. But to maximize the space, many of the major slides snake outdoors. There is a gaggle of multi-colored slide sections along the outside of the park's building that represent thousands more square feet.

Because the slides that extend outside are enclosed tubes, and the water that courses through them is heated, riders stay relatively toasty even when the outdoor temperature plummets and snow is falling. (This is the Pocono Mountains, after all.) Actually, that's half the fun of the indoor water park: When the weather outside is frightful, bathing suit-clad visitors can frolic inside the delightful, climate-controlled, 84-degree park like it's a balmy summer day.

Visitors inside the park can't see what awaits them on many of the rides, since the slides go outside and are hidden from view. The mystery helps add to the thrills.

The sheer quantity and variety of attractions are impressive. To name a few: Cheetah Race is a four-lane, mat-racing slide with a digital leader board that tracks finish times. The Wild Wildebeest is an airtime-filled slide that sends passengers in four-person cloverleaf tubes soaring up and down a half pipe wall. The Screaming Hyena starts way up in the building's rafters in a nearly vertical capsule. After a 3-2-1 countdown, a trap door opens to release sliders into a pulse-quickening drop (and a wedgie-inducing splashdown at the bottom).

Everybody in the pool

Not all of the attractions aim to get adrenaline pumping. Among the more sedate alternatives is an extra-long lazy river. "Transpotainment" conveyor belts connect the park's original lazy river with a second one that opened with the expansion. There are also two indoor-to-outdoor whirlpools. It's especially lovely to head outside in the bubbling hot water during a snowstorm.

The over-21 crowd can also enjoy an outdoor hot tub that doubles as a swim-up bar. As with other lounges throughout the resort, the bar specializes in oversized drinks. Adults would also appreciate the Kalahari's full-service spa, its 100,000-square-foot convention center, and its restaurants. The Double Cut Grill is a surprisingly sophisticated steakhouse with especially tasty charcoal-fired fare. It opened, along with Sortino's Italian Kitchen, as part of the 2017 expansion. The food at Sortino's was also quite good, although it commits a cardinal sin for an Italian restaurant by not serving bread with its meals.

On the other end of the age spectrum, the Kalahari caters to younger children with Splashdown Safari, a large interactive water play structure with smaller slides, net crawls, water guns, and a tipping bucket that unleashes a torrent of water every few minutes. Really tiny tots can also find activities geared just for them including water tables. That wasn't always the case at Kalahari Resorts.

The Poconos location is the third indoor water park resort for the family-operated company. The original Kalahari is in Wisconsin Dells, Wis., the water park capital of the world and the birthplace of the indoor water park phenomenon. There is also a resort in Sandusky, Ohio. At first, the Kalahari targeted families with children 10 and older, according to Todd Nelson, owner and president.

"About five years ago, we started having grandchildren," he says. That opened his eyes to an audience that his water parks weren't really serving. He got to work developing custom age-appropriate features, most of which were designed and built at the Kalahari's in-house theme shop. "We now have some of the best water park attractions for kids five and under in the industry," Nelson adds.

Great Wolf Resorts, a competitor, has long courted toddlers with its indoor water park hotels. It also has a location in the Pocono Mountains (as well as in Wisconsin Dells and Sandusky) along with Aquatopia, another major indoor water park hotel that is part of the Camelback Mountain ski resort. Nelson isn't concerned about saturating the market. "We feed off each other," he insists and says that the waterpark resorts have helped energize the once-thriving vacation destination. "The rebirth of the Pocono Mountains is well underway now."

The resorts may feed off one another, but having the country's largest indoor water park is clearly a matter of pride for Nelson. "We like to win," he says. There are sound business reasons for claiming the title as well. "The larger we are, the more fun we can offer, and the more people we can appeal to."

There is plenty of fun to be had at the Kalahari. In addition to the water park, there is a 40,000-square-foot family entertainment center with arcade games, mini-golf, bowling, and an interactive motion theater ride.

There is also The Arena, a free-roam virtual reality experience. Participants can choose Zombie Survival, a first-person shooting game, or Engineerium, a puzzle adventure. In both cases, players must work collaboratively in the virtual world. The graphics and game play were impressive. Of the two, Engineerium was trippier and included some mind-bending sequences that appeared to allow participants to defy gravity. The 15-minute experience costs $25.

It's a jungle in there

The African-themed resort is filled with artifacts, stunning photos, and other items that Nelson and his family have personally gathered during the many trips they have taken to the continent. It helps give the Kalahari a sense of authenticity.

With the expansion, the hotel now offers almost 1,000 rooms. There are a variety of configurations, including suites with up to five bedrooms. There is a honeymoon suite with a heart-shaped whirlpool (this is the Pocono Mountains, after all). A Kids Bunk Room, which sleeps up to four, costs about $360 to $500 per night on weekends, depending on the time of year. Mid-week prices are lower. The room rates include water park passes for all hotel guests.

Checkout is 11 a.m., but hotel guests can use the water park all day on the day they depart. That can make for big crowds and long lines in the water park on Saturdays as guests leaving and arriving converge on the water slides. Unlike Great Wolf Lodge and Aquatopia, which restricts water park admission to hotel guests, the Kalahari does offer day passes, based on availability, to the general public. That can add to the crowds, although even on weekends the lines are fairly short early and late in the day. All-day water park passes for visitors not staying in the hotel cost $75.

Nelson says he's not done expanding his Poconos resort. There are plans to double the size of the convention center, increase the outdoor water park attractions, and add an indoor theme park. He is not sure whether he will continue developing the hill that he's constructed outside the hotel and turn it into a snow tubing attraction or use the land for something else.

There are more Kalahari resorts on the way as well. The company announced that it would open its fourth location in 2020 in Round Rock, Texas, near Austin. Nelson says it would be his largest resort and include amusement park rides such as roller coasters and an exhibit about Nelson Mandela in addition to indoor and outdoor water parks.