  Tor is an anonymity network that allows offering and accessing various kinds
of resources, known as hidden services, while guaranteeing sender and receiver
anonymity. The Tor web is the set of web resources that exist on the Tor
network, and Tor websites are part of the so-called dark web. Recent research
works have evaluated Tor security, evolution over time, and thematic
organization. Nevertheless, few information are available about the structure
of the graph defined by the network of Tor websites. The limited number of Tor
entry points that can be used to crawl the network renders the study of this
graph far from being simple. In this paper we aim at better characterizing the
Tor Web by analyzing three crawling datasets collected over a five-month time
frame. On the one hand, we extensively study the global properties of the Tor
Web, considering two different graph representations and verifying the impact
of Tor's renowned volatility. We present an in depth investigation of the key
features of the Tor Web graph showing what makes it different from the surface
Web graph. On the other hand, we assess the relationship between contents and
structural features. We analyse the local properties of the Tor Web to better
characterize the role different services play in the network and to understand
to which extent topological features are related to the contents of a service.
