In order to store data in multi-layer form, the beam is "focused to different depths inside the photopolymer layer," which means that the beam can actually be raised and lowered to write to different altitudes of the three-dimensional holograohic grid.
As you might expect, the project is bullish on its prospects. The discs are predicted to be inexpensive to produce, which the project hopes will translate into manufacturer interest. Of course, there’s no word on what the recording devices would cost, and that’s a major part of the equation.
Blu-ray discs currently store 50GB of data on a dual-layer disc, whereas HD-DVD can store 30GB on its dual-layer offering. Both formats have greater storage potential thanks to the possibility of adding additional layers. By 2010, we expect to see 100GB and 60GB Blu-ray and HD DVD discs (respectively) available for recording use on PCs.
Microholographic discs (MHD) are transparent and are the same physical size as CDs and DVDs, but the discs being created by the Microholas Project effectively have ten layers with five different wavelengths. The "prototype" discs are recordable and sport a 50Mb/s data rate, but the group expects an "Advanced Device" with 1TB of storage and data transfer speeds in excess of 200Mb/s by 2010."