There are three types of buyers for the world’s rarest Scotch whiskies: drinkers, collectors, and investors. Andy Simpson is all three.

“Today’s collectors provide liquid for tomorrow’s drinkers,” says Simpson. What he means by that is there’s a symbiotic relationship between the three: if people drank all the Scotch made in the past, there would be none to sell in the future.

Last year, U.K. auctions sold 107,890 bottles of Single Malt Scotch Whisky, more than triple the total in 2014. It turns out Simpson isn’t just a rare whisky collector, he’s also the data-obsessed co-founder of Rare Whisky 101, which tracks the value and volume of whiskies sold at U.K. auctions.

Even amid the stress-inducing Brexit, Scotch whiskies are toasting massive valuation gains. Rare Whisky 101’s Apex 1000—a dynamic index that measures the hammer price of the 1,000 best-performing Single Malt Scotch sold at auction—has increased in value by nearly 163% over the past four years, besting the Dow Jones Industrial Average, Brent crude, gold, and even the stock performance of Diageo, the world’s largest liquor company deo .

“Scotch has cachet,” says Simpson. But the boon is also due to the increasing popularity and proliferation of online auction sales, making it easier for collectors and investors outside the U.K. to buy into a craze that’s reminiscent of Dutch tulip mania.

In 2012, auctions sold just 14,150 bottles of rare Scotch at a total value of 2.9 million British pounds. By comparison, in March 2019, 13,347 bottles sold at 5.5 million pounds. Nearly all transactions occurred online.

Edrington dominates the Scotch auction market: the top two sellers by volume are the privately-held company’s The Macallan and Highland Park brands. LVMH’s Ardbeg lvmhf , Remy Cointreu’s Bruichladdich, and Beam Suntory’s Bowmore round out the top five.

“[Macallan] is the whisky that made me like Scotch—it has that universal, approachable flavor profile,” says Nicolas Villalon. He’s paid to say things like that these days, as he serves as The Macallan’s brand education and prestige, helping manage the brand’s rare whisky portfolio.

But to his credit, Villalon is right: Macallan makes delicious, well-regarded whiskies. “Macallan makes drinkers go nuts,” says Simpson. “They have a fantastic back catalogue. There’s always a new release, something from 30 years ago a collector doesn’t have that excites them.”

Macallan keeps breaking records at the auction house due to the 60-Year-Old Macallan 1926, which is essentially the New England Patriots of the Scotch whisky world. Bottled in 1986, only 40 bottles were released from a single cask with unique labels created by artists like Sir Peter Blake (co-creator of The Beatles’ Sgt Pepper’s Lonely Hearts Club Band album cover art) and Italian artist Valerio Adami. Each designed labels for 12 bottles of Macallan 1926.

Last April, an airport retailer broke records when it sold two bottles of The Macallan 1926—one with labels from each artist—for $600,000 apiece. By the fall, the record surged to over $1.5 million when a single bottle of The Macallan 1926 sold at Christie’s, boosted by the allure of a one-of-a-kind, hand-painted label by Irish artist Michael Dillon.

The Macallan has sought to democratize the rare whisky experience, selling a limited edition series starting in 2015 for as little as $99—a bargain compared to Macallan’s higher end portfolio that is priced at $500 and up.

“When we started to make the Edition Series, it wasn’t produced to be collected,” says Villalon. And yet, it is: the 2015 version is selling for over $1,000 on secondary markets.

Heady growth for Scotch has slowed down in recent months, with the secondary market essentially flattening this year. And insiders say there’s no new bottle on the block that’s expected to shatter a new record. Japanese whiskies have actually outperformed Scotch in recent months.

Counterfeits are another challenge, with at least 32 fakes sold at auctions over the past decade—though there is no way of knowing for sure how many fakes have been sold. Brands are adding anti-counterfeit devices to fight the fakes. Macallan, for example, has added holograms to bottles and other measures the brand won’t disclose.

But the secondary market’s biggest threats may be the so-called “double Ds,” bottles that are irresponsibly dropped or drunk. “After a night out and having one too many, that rare bottle gets opened,” says Simpson, “And sometimes I think, ‘‘Eh, I should have just had the Johnnie Walker Red Label.’”

Simpson has been a collector for three decades but his favorite bottle isn’t one with a fancy label. His star whisky is a “plain, drab” bottle from Glenburgie, which he still thinks is pretty fabulous.

So while he’s a drinker at heart, the collector in him is more powerful. “The virtue of whisky is that it should be opened and drunk,” says Simpson. “But I’m never going to find out what’s in that bottle.”