Read the README on GitHub, with the 2009 drawing and the scripts that redraw it.
An idea Michael thought up after reading K. Eric Drexler's Nanosystems: bits written as the stacking order of close-packed atomic layers. Two outer positions carry 0 and 1, and a spacer layer sits between every bit. Drag the view, or use the Turn, Tilt and Roll sliders, to see it from any side.
site Asite Bsite CA close-packed layer has to nest in the hollows of the layer below, so two neighbouring layers can never sit on the same site. With B between every bit, 0 = A and 1 = C, and any bit string is an allowed stack: 00 becomes A B A, never A A.
Each stack is one element, all silicon or all carbon. Both crystallise as diamond, in layers that sit on A, B or C sites. Left alone they repeat A B C, but a layer on another site, a stacking fault, is a known defect in both, and both have hexagonal forms stacked A B A B. Here every bit is a deliberate choice of site. The panel's figures use each element's own spacing: silicon 0.384 nm between atoms in a layer and 0.314 nm per layer, carbon 0.252 nm and 0.206 nm. Each ball stands for a pair of atoms, since both crystals hold two at every site. EB per cm³ is exabytes if stacks this size were packed side by side through a cubic centimetre.
Seen from the cut side face, the three sites sit at three different positions across the face: A at 0, B a third of the way along and C two thirds. An atomic force microscope tip run up the stack meets each layer's atom row at one of those three places, so the zig-zag it traces is the code. Read with AFM shows the tip doing that and decodes the trace.
DNA stores 2 bits per base pair, 0.34 nm apart, so about 0.17 nm per bit. That is denser along its length than the carbon stack at 0.41 nm. But DNA is read by sequencing it, not by running a tip along it, and the stack is a solid crystal.
Drexler's Engines of Creation and Nanosystems imagined machines that place atoms one at a time. Writing a stack like this needs exactly that: one layer at a time, each on a chosen site. Reading it back needs atomic resolution on the side face, which atomic force microscopes and electron microscopes both reach today.