  The structure of the Heisenberg evolution of operators plays a key role in
explaining diverse processes in quantum many-body systems. In this paper, we
discuss a new universal feature of operator evolution: an operator can develop
a void during its evolution, where its nontrivial parts become separated by a
region of identity operators. Such processes are present in both integrable and
chaotic systems, and are required by unitarity. We show that void formation has
important implications for unitarity of entanglement growth and generation of
mutual information and multipartite entanglement. We study explicitly the
probability distributions of void formation in a number of unitary circuit
models, and conjecture that in a quantum chaotic system the distribution is
given by the one we find in random unitary circuits, which we refer to as the
random void distribution. We also show that random unitary circuits lead to the
same pattern of entanglement growth for multiple intervals as in
(1+1)-dimensional holographic CFTs after a global quench, which can be used to
argue that the random void distribution leads to maximal entanglement growth.
