These 4 mansions have incredible secrets buried underneath them

These mansions are not what they seem, hiding secrets from a massive underground spa to a covert man cave buried under the front lawn. Take a look inside four homes with incredible subterranean lairs that you have to see to believe.

Subterranean secrets in St. Moritz

From the outside, this chalet in St. Moritz, Switzerland looks like a relatively modest one-story home. But in reality, it's a behemoth with seven levels, most of which are hidden underground.

The deceivingly exterior of a St. Moritz home that has seven levels, most of them hidden underground.

Source: Senada Adzem

When the home hit the market in 2017, its for $185 million price tag made it the most expensive home for sale in Switzerland.

Master bedroom

Source: Senada Adzem

The home's most extraordinary amenity is six levels down by elevator, built into the rock below: a massive subterranean spa.

"This place was built to make a billionaire's jaw drop," said Douglas Elliman/Knight Frank global agent Senada Adzem who represented the seller.

Off the elevator there's a giant pool with a Venetian sculpture suspended above it.

The subterranean pool with Venetian art installation hanging overhead.

Source: Senada Adzem

Right next to the pool is a huge candle-lit hot tub.

The steam-covered hot tub overlooking the pool.

Source: Senada Adzem

Steps away from the hot bath is steel-clad ice chamber.

Ice chamber

Source: Senada Adzem

There's also a Himalayan salt room with glowing rock walls.

Lounge chairs inside the glowing Himalayan salt room.

Source: Senada Adzem

Adzem says the owner declined two offers and ultimately decided to keep the home.

Moroccan surprise

In Rolling Hills, California is a mansion called "Hacienda de la Paz" that also looks deceivingly small from the outside. But don't let the villa's single-story exterior fool you.

Exterior view of the modest looking single-story villa "Hacienda de la paz"

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This massive homes spans 51,000 square feet, much of which falls below ground. It took the original owner, food magnate John Blazevich, almost two decades to build the nine-bedroom, 25-bath residence. Community rules restricted him from building above the first level , so he built down...way down.

And if you know where to find it, there's a secret passageway that leads to a mind-blowing subterranean lair more than 20 feet below ground-level.

CNBC's Robert Frank and Alex McLeod walk down the hidden staircase to the home's lower levels.

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One of the lower levels includes a 10,000-square-foot Moroccan-themed spa with an authentic Hammam and elaborate archways that lead to a stunning lap pool.

The luxurious lap pool hidden deep below "Hacienda de la Paz"

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The more than seven-acre estate has another pool in the sun drenched backyard.

Backyard pool

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On another below-ground level is a tennis court built into a 15,000-square-foot space that was designed to convert into a subterranean ballroom. (If you prefer playing on a clay court, there's one of those out back as well, with views of the Pacific.)

Subterranean tennis court that can be turned into a ballroom.

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Once listed for $53 million, the mega-home sat on the market for several years with no takers. Blazevitch eventually put it up for auction in 2018 when it sold for a fraction of the asking price, bringing in just $22.4 million.

Covert man cave

The former CEO of Patron Spirits built an extraordinary beach-front mansion in Manalapan, Florida. The 40,000-square-foot home has nine bedrooms, 16 baths and two levels that you can see from outside. But there are more below ground.

Source: Andy Frame Photography

What you can't see when you look at the exterior is what's buried under the front lawn.

The living room has 30 foot windows that are filled with ocean views.

Source: Andy Frame Photography

A doorway off the lower part of the estate's driveway leads to a covert man cave.

The space includes an underground lounge and bar, as you might guess it's stocked with tequila. Next to the bar is where Brown, who is also a race car driver, parked his Ferrari 458 and Le Mans prototype race car. He's also put a racing simulator and giant TV down here.

Underground bar with Brown's race car parked along side it.

Source: Andy Frame Photography

A waterfall spills into the pool in the home's backyard.

Source: Andy Frame Photography

In 2017 the home was on the market for the $48.9 million. It sold that same year for $40 million.