  With nearly 2.5m users, onion services have become the prominent part of the
darkweb. Over the last five years alone, the number of onion domains has
increased 20x, reaching more than 700k unique domains in January 2022. As onion
services host various types of illicit content, they have become a valuable
resource for darkweb research and an integral part of e-crime investigation and
threat intelligence. However, this content is largely un-indexed by today's
search engines and researchers have to rely on outdated or manually-collected
datasets that are limited in scale, scope, or both.
  To tackle this problem, we built Dizzy: An open-source crawling and analysis
system for onion services. Dizzy implements novel techniques to explore,
update, check, and classify onion services at scale, without overwhelming the
Tor network. We deployed Dizzy in April 2021 and used it to analyze more than
63.3m crawled onion webpages, focusing on domain operations, web content,
cryptocurrency usage, and web graph. Our main findings show that onion services
are unreliable due to their high churn rate, have a relatively small number of
reachable domains that are often similar and illicit, enjoy a growing
underground cryptocurrency economy, and have a graph that is relatively
tightly-knit to, but topologically different from, the regular web's graph.
