“We only know her as Susan,” the friendly woman on the other end of the phone told me. I had called a number in the small town of Pahrump, Nevada, which sits equidistant between Death Valley and Las Vegas, an hour away from both. I was trying to find the founder behind Aztec Secret Indian Healing Clay, the most reviewed beauty product on Amazon right now. “We never know when she’ll be here. She travels a lot.”

Aztec Secret, if you’ve never heard of it, is always rated somewhere between first- and sixth-most popular on Amazon’s beauty bestseller page. It has a whopping 11,500 customer reviews, over 10,000 of which are 4- or 5-star glowing reviews. This dwarfs the next most-reviewed item, a Crest teeth whitening kit, by morethan 3,600 responses.

The tub exhorts you, in all-caps red print, to FEEL YOUR FACE PULSATE.

In the world of $200 skincare and scientifically tested ingredients, the product that has inspired thousands of people to pen four-paragraph odes is a humble one. It’s a 100 percent bentonite clay powder that you mix with water or apple cider vinegar to use as a face mask. (More on this later.) It costs about $10 and comes in a one-pound tub with a label that looks like a kid made it for a school project with a first-generation color printer. Some witchy aspects, such as that you’re not allowed to mix it in a metal bowl, add to the intrigue. The tub exhorts you, in all-caps red print, to FEEL YOUR FACE PULSATE. At the bottom of the label, almost as an afterthought, it says “Men love it, too.”

According to the company’s website — which looks like it hails from the same era as that kid’s printer — the clay comes from Death Valley, “where it is sun-dried for up to six months in temperatures that sometimes reach 134 degrees.” Bentonite clay is derived from volcanic ash, and the long list of substances it contains reads like the periodic table of elements: potassium, cesium, ytterbium, and on and on.

Although it’s commonly found in commercial clay mask preparations, the uses for bentonite clay go far beyond cleaning out your pores: It’s used in calamine lotion, to remove impurities from food oils, as a fabric softener in laundry detergent, to form animal feed pellets, and — my favorite — it’s the ingredient that causes cat litter to clump. It’s also the type of clay that Shailene Woodley slurps down, though her preferred brand isn’t Aztec Secret. (To be clear, Aztec only markets its clay as a topical ingredient. While there are tons of wellness sites that tout the many supposed benefits of clay for detoxing, definitely don’t eat this.)

Aztec Secret has become an unlikely phenomenon, with no advertising and no savvy Instagram infiltration, like all those teatox brands. So how did this homely product become such a hit? I started first with the Friends-era website, intending to send an email to the company in hopes of finding a founder. There was no email address listed on the site, only a PO box, phone, and fax number. I found a street address at a few directory sites online, but a view via Google Street View showed me literal tumbleweeds, not even a building. There’s also a PDF order form for ordering bulk quantities of Aztec Secret, payable only by check or money order, and to be mailed in a proper envelope — no e-commerce here.

When I called the number and explained that I was a reporter wanting to speak to the founder, that’s when I discovered surname-less Susan. Could I possibly email her? “No, you have to fax her.” The employee gave me a fax number different from the one listed on the website. I then did something I haven’t done in ten years: I made a cover sheet and faxed something. After calling back for a week, I was finally told nicely that Susan didn’t want to speak to me. Not totally shocked, I resorted to some internet archeology instead to trace Aztec Secret’s rise to Amazon beauty legend.

Unsurprisingly, Aztec likely got its start in health food stores and Whole Foods, though its earliest Amazon review dates to 2005, a date consistent with Susan’s technology. There are only two reviews in 2005, then not another one until 2007. The product popped up on an acne message board in 2006, and that’s also when it made its first appearance on Makeup Alley, where a user noted that she bought a tub for $5 at Whole Foods. In 2008, Amazon reviews started picking up a bit more, and ramped up slowly from there.

Which brings us to the modern iteration of Aztec Secret. In 2012, Aztec started showing up on Reddit and blogs, including this one where the writer says an older woman in a health food store told her she’d been using it for 20 years (!!). Mainstream publications and websites then started steadily covering Aztec.

Allure’s “Beauty Blogger of the Year” wrote about it on the magazine’s blog in 2012, XOJane called it out in 2013, and Into the Gloss mentioned it in 2014. This period of time is also when YouTube and Instagram influencers started to emerge, and wellness, the concept of detoxing, and DIY beauty treatments started to really trend. Aztec, in all its earthy, muddy, “natural” glory, really fit into the moment.

In 2015, Aztec had its biggest year yet. Celebs like Mindy Kaling name-checked it, and it got more play in the media. Then, searches for the product suddenly spiked during the week of November 22nd, according to Google Trends. It’s been steadily increasing ever since. Was it because everyone was home detoxing during Thanksgiving week in 2015 or… ?

Nope, it was YouTube. Two beauty vloggers posted videos four days apart during that time. First, Toronto-based Samantha Jane, who has a modest-by-YouTube-standards 22,000 followers, posted a video review on November 19th that got over 200,000 views, making it the most-viewed video on her channel. Then Kathleen Lights, who has almost three million subscribers and is huge on Instagram as well, posted one on the 23rd, which has almost 570,000 views to date. In the video, she called it “the most intense mask in the history of the world.”

This brings us back to the experience of using this mask, which people are still enthusiastically buying, at least if January 2017 Amazon reviews and another spike the first week of the month are any indication. Around 87 percent of both Amazon and Makeup Alley reviewers felt positively about Aztec Secret.

It’s not surprising that so many people have bought it, particularly those struggling with acne, once you start reading reviews like this: “I have only been on this product for three nights and my scars are evening out unlike anything before. Amazing. Even my mother is shocked with photos as she's has[sic] a front row seat to my acne struggle and one of the few people to see my skin natural through this time.”

Sometimes I put glop on my face for beauty reasons. This one was cheapo and fun to mix and really worked. I don't get paid by them or anything! Just have mega oily skin and @clarisshair and @jankinjo recommended this on book tour. It works! Plus it makes me look frightening. A photo posted by Mindy Kaling (@mindykaling) on Sep 30, 2015 at 6:43am PDT

It’s really fun to use this product, and, combined with the cheap price point, that makes it a winner. The simplest way to use it is to mix it with either water or apple cider vinegar. The label actually recommends mixing it with apple cider vinegar, and numerous people online theorize that the vinegar, which is very acidic, helps neutralize the pH of the clay, though I can’t find an authoritative source to help me confirm that this is actually true. However, someone on Reddit’s Skincare Addiction subreddit said it, so that’s good enough for me.

The label also recommends not using a metal bowl or spoon to mix it, because something about ions. Allure asked cosmetic chemist Randy Schueller, also my favorite go-to BS-buster, about this, and he said, “A number of websites say that contact with metal will cause the clay to become less effective or lose its 'magnetic charge.’ This is simply not true. Bentonite clay contains salts that create an ionic charge when dissolved in water, but metal has no effect on this.” But the good news from a dermatologist interviewed in that same article is that bentonite clay really can help clean gunk out of your face.

I mixed two small batches, one with water and one with apple cider vinegar. Aesthetically, the resulting paste made with the vinegar was the hands-down winner. The mixture with water looked like, well, the same chunks I clean out of my cat’s litter box. It really didn’t mix smoothly. The vinegar also provided a satisfying sizzling, bubbling moment in the bowl.

The vinegar mixture also felt better on my skin. I patch tested both on the back of my hands before I put it anywhere near my face. While there aren’t as many negative as positive reviews of Aztec Secret, the ones that are there will scare the shit out of you. “PLEASE listen to me. This product ruined my skin.” And “I AM NOT FINE! I am in pain and I am furious! This stuff is going back!! STAY FAR AWAY!” The water mixture actually burned a little while the vinegar mixture didn’t at all. So I slathered a bit on my nose and a few breakout-prone areas and waited.

“PLEASE listen to me. This product ruined my skin.”

As the clay dries, it tightens, and yes, pulsates. But if you’re a clay mask vet, this sensation won’t be shocking to you. My skin felt pretty dry and tight afterwards, but the pores on my nose honestly seemed to have shrunk (this isn’t physiologically possible but still) and there were many fewer blackheads.

But this clay isn’t all fun bubbles and blackhead busting. Many users, including Racked’s own favorite beauty writer Claire Carusillo (who wrote about it in her newsletter), have complained about how drying it is. The tub recommends using it only once a week. It also clogs drains, which makes it a pain to rinse off. I lined my sink with paper towels to catch the residue. Also, even among people who give it overall positive reviews, it seems to frequently cause redness and irritation, so definitely test it on a small area first. Finally, and perhaps most importantly, bentonite clay can contain lead, arsenic, or bacteria, according to a 2014 New York Times article. This was confirmed again last year when the FDA found high lead levels in a brand of clay called Bentonite Me Baby. It’s unclear how bad it is to apply topically, but it bears repeating here: Please, don’t eat it.

If you’re an Aztec Secret late adopter and want to try it, it’s sold by any of 50 different “storefronts” on Amazon, and prices there are pretty consistently $10 plus or minus a few cents. You can also purchase it at Thrive Market for less than $5 and the Vitamin Shoppe for about $7.50, not to mention at many local beauty and health food stores. There are tons of other brands that also offer 100 percent bentonite clay, too, but none that have resonated, or presumably sold, like Aztec Secret.

I recently went to a beauty launch for a $50 moisturizer that the founder bragged had sold out within its first week at Sephora. He openly acknowledged that his company had flooded social media, public transportation, and magazines with ads. Aztec Secret did it without modern communication methods or even a founder with a last name. Kudos to you, Susan.

Update: Thanks to a tip on Twitter from Kathryn Swartz Rees, I was able to find the name of the family who owns Aztec via business filings. Aztec was founded in 1986 by an Indiana native named Mary Roman. She died in 2012, and according to an obituary in the Pahrump Valley Times, was a champion water skier and loved to play video poker and "wear hats." Her daughter Denise and son Patrick now run the business, at least according to records.

In light of this new information, I called Aztec again, this time asking to speak to Denise Roman or anyone else from the family. The same woman answered, and said they were traveling and they had no idea when they'd be back. Could I email her? "No, we don't own a computer." OK, is Susan there? Does a Susan even work there? "Yes, but she's not here either."

I still don't know who Susan is.