I have been riding a set of Alien Workshop Microlites wheels for the past 6 months. They are light (due to the lightweight inner air core), hard, no flat-spots and then all of a sudden this happened to one of the wheels.

It seems as though the glue holding the urethane core to the urethane shell (outer part of the wheel) has lost its adhesiveness. In the third picture, you can see a small hole created by a tiny air bubble during the manufacturing process of the wheel (poor quality control) this may have aided in the outer shell coming loose and splitting. The three other wheels are still in tact.

Luckily I wasn't riding my skateboard on the expressway or I could have been thrown under a car. Although it would probably take a bus or a tank to crush me.

*When I bought the wheels I noticed the small hole on the wheel above but didn't take too much notice of it. Big mistake! Don't buy any wheels that have tiny holes in them.