After more than four years and two giant law enforcement busts, the Dark Web's drug market is still just as robust as it was during the Silk Road's heyday. In fact, according to a new study, it's now moving well over $100 million of illegal substances a year, and it's recovering from every new scam-induced setback and government crackdown faster than the last one.

Those are a few of the lessons from a deep-dive study into Dark Web black market sales statistics that a pair of Carnegie Mellon researchers plan to present at the Usenix Security conference Wednesday. From 2013 to early 2015, they used automated software to "scrape" the visible contents of 35 Dark Web markets, counting the number of feedback ratings that the major drug sites require buyers to leave after each sale and then multiplying them by the purchased item's listed price to gauge total sales volumes. The result is the most comprehensive—if still not fully complete—picture of how contraband narcotics sales have waned and waxed during the short history of the Dark Web's online economy.

The researchers' findings, which extend to January of this year, describe a market that's no longer explosively growing, as it did in the early days of Silk Road. (In 2012, for instance, the site doubled sellers and more than doubled sales in just six months, reaching $22 million in annual sales.) Instead, says Carnegie Mellon University computer science professor and longtime Dark Web-watcher Nicolas Cristin, it's now fluctuating between a rate of around $100 million and $180 million a year in total sales volume. "The market is relatively stable, with sales between 300,000 and 500,000 dollars a day," says Christin, who ran the study along with fellow CMU researcher Kyle Soska. "It’s not huge, but it’s not negligible, and it’s a lot bigger than we had reported in 2012."

'It's going to be a lot harder to get rid of these marketplaces than one would have thought.' Carnegie Mellon researcher Nicolas Christin

More surprising, perhaps, is that the Dark Web economy roughly maintains that sales volume even after major disasters like thefts, scams, takedowns, and arrests. According to the Carnegie Mellon data, the market quickly recovered after the Silk Road 2 market lost millions of dollars of users' bitcoins in an apparent hack or theft. Even law enforcement operations that remove entire marketplaces, as in last year's purge of half a dozen sites in the Europol/FBI investigation known as Operation Onymous, haven't dropped the market under $100 million in sales per year. "What we’ve seen is that, as a whole, the ecosystem is resilient to these adverse events," says Christin. "That shows it’s going to be a lot harder to get rid of these marketplaces than one would have thought."

The relatively limited impacts of these recent setbacks contrast starkly with earlier dings to the Dark Web drug economy. When the FBI shut down the original Silk Road and arrested its creator Ross Ulbricht in late 2013, for instance, the researchers believe the total sales volume of the Dark Web's contraband economy dropped far more drastically. And when the Silk Road successor Sheep Marketplace absconded with millions of buyers' and sellers' bitcoins two months later, the total sales of the internet drug market took another major hit. (In both of those cases, however, the researchers hadn't begun scraping smaller competing sites in the market early enough to exactly quantify those drops in total sales.)

Here's a chart that shows the full extent of the study's estimates of daily sales volume over time for seven major Dark Web markets:

The total sales volume over time for seven major Dark Web drug market sites. The total sales volume over time for seven major Dark Web drug market sites. Carnegie Mellon

The Carnegie Mellon study represents the best attempt yet to measure the size and trajectory of the drug markets that use tools like the anonymity software Tor and bitcoin to hide visitors' identities and evade law enforcement. Earlier attempts to gauge that economy had resorted to merely counting product listings on different sites, which was a flawed method, given that dealers often listed the same products on multiple sites, and some products sell more than others.

Even so, the Carnegie Mellon study ended prematurely in early 2015 due to the researchers' difficulties in scraping the two major market sites—Evolution and Agora—that survived after Operation Onymous. When that crackdown eliminated Evolution's competition, the site grew so big that Carnegie Mellon's software could no longer reliably scrape its full content, Christin says. Agora, meanwhile, suffered from downtime that also prevented it from being reliably measured.

That means the researchers can't assess what exactly happened after Evolution abruptly shut down in March and stole all its customers' and dealers' funds. (Christin speculates that Agora has likely absorbed most of the site's refugees.) But the researchers also note in their study that it may be too early to fully judge the longterm effects of Operation Onymous' darknet purge. "The jury is still out," says Christin. "We don’t have enough reliable data to assert whether it was a waste of time. But business picked up quite quickly—even more quickly than after the Silk Road takedown."

The study's sales data reveals a number of smaller points, too:

Although a few Dark Web dealers have sold huge amounts of narcotics, most vendors sell very little and don't make much money: 70 percent of them sold less than $1,000 worth of products , while only 2 percent sold more than $100,000. The researchers argue that means most dealers are competing only with retail-level street drug dealers, rather than wholesale operations.

, while only 2 percent sold more than $100,000. The researchers argue that means most dealers are competing only with retail-level street drug dealers, rather than wholesale operations. Dark Web market sellers are adopting more encryption tools. While they've been slow to start using so-called multi-signature transactions—a feature of bitcoin that could prevent the currency from being stolen by fraudsters or seized by cops—they've almost universally started communicating with the crypto software PGP. In the original Silk Road, as few as two out of three vendors posted a PGP key on their sales pages to allow buyers to send them encrypted messages, while earlier this year, 90 percent of Evolution and Agora sellers posted a key.

While they've been slow to start using so-called multi-signature transactions—a feature of bitcoin that could prevent the currency from being stolen by fraudsters or seized by cops—they've almost universally started communicating with the crypto software PGP. In the original Silk Road, as few as two out of three vendors posted a PGP key on their sales pages to allow buyers to send them encrypted messages, while earlier this year, 90 percent of Evolution and Agora sellers posted a key. The Dark Web's most popular drugs appear to be marijuana and MDMA, with each accounting for close to 25 percent of the total sales volume. Stimulants, including cocaine, at times accounted for another 25 percent of the market, but fell off in recent months, while prescription drugs, including Viagra and Oxycontin, have grown. Opioids like heroin account for less than 10 percent of the market's sales. Here's a full chart of sales volumes of different drug types over time:

Market share of different drug types over time. Market share of different drug types over time. Carnegie Mellon

The researchers end their paper with a plea for policymakers to rethink their attempts to simply shut down the Dark Web's markets. "It is not clear that take-downs will be effective; at least we have found no evidence they were," they write. Instead, they argue that an unchanging demand for narcotics drives the resilience of the clandestine digital drug trade, and the Dark Web may have the potential to reduce violence at the retail level of the drug trade. All of that, they say, deserves more scrutiny from lawmakers and law enforcement.

Read the researchers' full paper here.