Seashells are natural body armors with superior mechanical strength and ultra-high toughness compared with their major constituent counterparts. What building blocks and architecture render seashells such mechanical prowess? In this study, micro/nanoscale structural and mechanical characterization of conch shells (Busycon carica) has been carried out. Here we show direct evidence that the previously claimed single-crystal third-order lamellae - the basic building blocks in conch shells are essentially assembled with aragonite nanoparticles of the size ranging from 20 to 45 nm. The nanoparticle-constructed third-order lamellae are not brittle, but ductile. The three-order crossed-lamellar architecture interlocks cracks via crack deflection along the interfaces in a three-dimensional manner, thus confining the damage in a small region. The findings advance the understanding of the mystery of conch shell's mechanical robustness, provide additional design guidelines for developing bioinspired nanomaterials, and lay a constitutive foundation for modeling the deformation behavior of seashells. (C) 2013 Elsevier Inc. All rights reserved.