Untold riches are promised on Mystery Brand, a website that sells prize-filled “mystery boxes.” If you buy one of the digital boxes, some of which cost hundreds of dollars, you might only get a fidget spinner—or you might get a luxury sports car.

For just $100, users can win a box filled with rare Supreme streetwear. For only $12.99, they can win a Lamborghini, or even a $250 million mega-mansion billed as “the most expensive Los Angeles realty.”

Or at least that’s what some top YouTubers have been telling their young fans about the gambling site—with the video stars apparently seeing that as a gamble worth taking, especially after a dip in YouTube advertising rates.

Over the past week, hugely popular YouTube stars like Jake Paul and Bryan “Ricegum” Le have encouraged their fans to spend money on Mystery Brand, a previously little-known site that appears to be based in Poland.

In their videos, Paul and Le show themselves betting hundreds of dollars on the site for a chance to open a digital “box.” At first, they win only low-value prizes like fidget spinners or Converse sneakers. By the end of the video, though, they have won thousands of dollars worth of tech and clothing, like rare pairs of sneakers or Apple AirPods.

If they like the prize, the YouTube stars have it shipped to their house.

“Go to MysteryBrand.net if you want to win some Mystery Boxes!” Paul says in his video, surrounded by his winnings from the site.

Le, whose YouTube channel has more than 10 million subscribers, encourages his fans to spend money at Mystery Brand in his own video, “How I got AirPods for $4.”

“Open the boxes,” Le says. “Get something good.”

The contents of the boxes aren’t the only mysterious thing about Mystery Brand.

The $250 million “Most Expensive Los Angeles Realty” house, for example, is just a picture of a Bel Air mansion listed for $188 million. It’s not owned by Mystery Brand, which initially listed the supposed odds of winning the mansion—0.0000018 percent—before removing those odds.

Other prizes are just as strange. Some boxes offer prizes with names like “Icicle - site balance” or “Ginger man site balance,” illustrated with stock imagery but without any further description.

Mystery Brand’s terms of service appear to say that underage users are ineligible to receive prizes, or even their money back, as the site will “invalidate all the operations that were carried out by a person who has not attained the age of majority and to refuse to issue a winning product without any refund of spend value.”

Mystery Brand users might not even receive the items they believed they have won, according to another part of the terms of service.

“During using the services of the website You may encounter circumstances in which Your won items will not be received,” the document reads.

It’s not clear who owns Mystery Brand or where it’s based, although the site’s terms of service say it’s “subject to the laws and jurisdiction of Poland.”

Despite that, Mystery Brand has teamed up with YouTube stars like Paul and Le, whose channels are aimed at children.

Paul, for example, has acknowledged that the bulk of his fanbase is between 8 and 15 years old. In October, he was accused of violating FCC rules against marketing to children with his frequent calls for kids to buy his merchandise.

Le and Paul aren’t the only child-focused YouTube channels that are promoting Mystery Brand. Guava Juice, a popular kids’ channel with its own line of toys, also made a video promoting spending money on Mystery Brand.

Morgan “Morgz” Hudson, a teenage British YouTube prankster who has said in the past that his videos are aimed at kids, posted his own Mystery Brand video with an affiliate link to the site, meaning he’ll receive money when his fans spend money on Mystery Box.

All four YouTube channels, as well as Mystery Brand, didn’t respond to requests for comment.

The sponsored videos have also been filleted by rival YouTubers, with one detractor claiming that Le and Paul were “finished.”

In Le’s video, he wins a pair of Off-White Nike Air Max 97’s on the site. But when Le later opens the box that he supposedly received from Mystery Brand, he pulls out a different, more valuable pair of shoes. On Reddit, Mystery Brand customers have complained that they never received prizes they won on the site.

Daniel Keem, a YouTube drama vlogger who goes by the handle “Keemstar,” claimed on Twitter on Tuesday that he had turned down $100,000 to promote Mystery Brand on his own channel.

“I was offered $100k to do the same & almost took the cash. (But didn’t),” Keem tweeted. “So I can’t go that hard on them.”