Whilst Blu-ray still hasn't fully embedded itself as the mainstream optical storage of choice, technology keeps on moving forward and efforts to push the barriers of limitation are always in place.
With that said, Pioneer has successfully found a way to put no less than 16 layers on what looks like a standard optical disc. What makes this even more impressive is that each layer is the same size as a standard Blu-ray's; 25GB.
Pioneer said the biggest challenge in accomplishing this was finding a way to stop crosstalk/interference between each layer as it becomes increasingly difficult to obtain a clear signal when more layers are packed onto the disc.
For multilayer optical discs, it has been difficult to obtain clear signals from each recording layer in a stable manner due to crosstalk from adjacent layers and transmission loss. Utilizing the optical disc production technology that it has developed in the DVD field, Pioneer solved these problems by, among other things, using a disc structure that can reduce crosstalk from adjacent layers, resulting in a 16-layer optical disc that can playback high-quality signals from every layer.
As for the read-out system, Pioneer achieved stability in the playback of recorded signals by employing a wide-range spherical aberration compensator and light-receiving element that can read out weak signals at a high signal-to-noise ratio in the optical pick-up mechanism. Since the optical specifications of the objective lens, such as NA (Numerical Aperture)*2, are the same as those for the existing BD discs, it is possible to maintain compatibility between the new 16-layer optical disc and the BD discs.
More details here.