Sprawled across a vast site on the river Rhine in Germany is a small city built from glittering steel: the headquarters of chemical giant BASF. Boasting a daytime population of about 50,000 people, it is criss-crossed by a grid of streets bearing names that commemorate the company's stock in trade: Methanolstrasse, Ammoniakstrasse, Gasstrasse.Over the past two years, a small fleet of delivery vans and cars has clocked up thousands of kilometres on these streets while carrying a big secret: fuel tanks packed with an unusual crystalline material that is riddled with pores roughly a nanometre wide. Inside these pores sit methane molecules arrayed in neat stacks, ready to fuel the vans' combustion engines.The crystals are metal-organic frameworks (MOFs), molecular scaffolds made up of metal-containing nodes linked by carbon-based struts THE HOLE STORY Swiss-cheese-like materials called metal-organic frameworks have long promised to improve gas storage, separation and catalysis. Now they are coming of age.