At first glance, the crystal looks like diamond, said Dr. Omar M. Yaghi, a professor of chemistry at the University of Michigan.

But the crystal, colorless, is light. A cube of the material one inch on a side weighs about a twentieth of an ounce. To the eye, it looks solid, but a microscope would reveal a Swiss cheese of microscopic holes. The molecules in the crystal stack so loosely that that 95 percent of the volume is nothing, empty space.

That is why Dr. Yaghi said, ''We think it's going to be very useful.''

A crystal that is mostly empty is useful for storing stuff -- hydrogen, in particular.

The compound is prosaically named Metal-Organic Framework No. 177, MOF-177 for short. Following in the steps of MOF-1 through MOF-176, it or one of its successors could become the fuel tank of the future.