Wireless sensor networks (WSNs) have become pervasive and are used for a plethora of applications and services. They are usually deployed with specific applications and services; thereby precluding their re-use when other applications and services are contemplated. This can inevitably lead to the proliferation of redundant WSN deployments. Virtualization is a technology that can aid in tackling this issue. It enables the sharing of resources/infrastructures by multiple independent entities. This position paper proposes a novel multi-layer architecture for WSN virtualization and identifies the research challenges. Related work is also discussed. We illustrate the potential of the architecture by applying it to a scenario in which WSNs are shared for fire monitoring.