  For two decades now, the importance of microstructure in the mechanical
behaviour of a large collection of grains has been a topic of intense research.
Many approaches have been developed to study the microstructure in granular
materials, based on local or mesoscopic measures or using ideas from diverse
fields like percolation theory, complex networks and persistent homology.
However, we do not fully understand the role of microstructure in the
mechanical behaviour of simple and commonplace packings like a pile of sand or
box of grains. Here, by characterizing the spatial variation of local
microstructure, we uncover intriguing large-scale spatial patterns in particle
packings deposited under the influence of gravity. We detail the emergence of
these patterns, and provide a unified fundamental explanation of classic and
puzzling mechanical behaviours of granular materials like central stress
minimum and Janssen effect. Further, we show the striking dependence of spatial
structure on the history of preparation, size distribution of particles and
boundary conditions. Our study reveals the existence of global spatial
structure in a locally disordered media, and elucidates its significance in the
mechanical behaviour of granular materials.
