  The fractionalization of quantum numbers in interacting quantum-many body
systems is a central motif in condensed matter physics with prominent examples
including the fractionalization of the electron in quantum Hall liquids or the
emergence of magnetic monopoles in spin-ice materials. Here we discuss the
fractionalization of magnetic moments in three-dimensional Kitaev models into
Majorana fermions (and a $\mathbb Z_2$ gauge field) and their emergent
collective behavior. We analytically demonstrate that the Majorana fermions
form a Weyl superconductor for the Kitaev model on the recently synthesized
hyperhoneycomb structure of $\beta$-Li$_2$IrO$_3$ when applying a magnetic
field. We characterize the topologically protected bulk and surface features of
this state, which we dub a Weyl spin liquid, including thermodynamic and
transport signatures.
