84,000 neurons, 523 million synapses and 4 kilometres of axons. All of this is hidden within 1 cubic millimetre of mouse brain, as revealed by the detailed map that research consortium Machine Intelligence from Cortical Networks presented on 9 April in a series of papers in Nature and Nature Methods. The accompanying image represents only a portion of that data, showing around a thousand brain cells in which each neuron has been assigned its own (arbitrary) colour. The new map reveals the structure and connections of all the brain cells. Measurements of neuronal activity, electron microscopy images and artificial intelligence made the map possible. (AK)