Tor hidden services allow you to make TCP services available while keeping your servers location hidden via the Tor anonymity network. Hidden services such as the Silk Road marketplace receive more than 60,000 unique user sessions a day. Tor hidden service directories (HSDir) receive a subset of hidden service look-ups from users, allowing them to map relative popularity/ usage of hidden services. An adversary with minimal resources can carry out complete DoS attacks of Tor hidden. services by positioning them in a particular part of the router list.”]
Source: https://donncha.is/2013/05/trawling-tor-hidden-services/