  Many complex systems may be described not by one, but by a number of complex
networks mapped one on the other in a multilayer structure. The interactions
and dependencies between these layers cause that what is true for a distinct
single layer does not necessarily reflect well the state of the entire system.
In this paper we study the robustness of three real-life examples of two-layer
complex systems that come from the fields of communication (the Internet),
transportation (the European railway system) and biology (the human brain). In
order to cover the whole range of features specific to these systems, we focus
on two extreme policies of system's response to failures, no rerouting and full
rerouting. Our main finding is that multilayer systems are much more vulnerable
to errors and intentional attacks than they seem to be from a single layer
perspective.
