[brummm]

Deeper. The only significance of the Planck length is that at that scale quantum effects and gravitational effects matter equally. Essentially that's where our current theories break down.

Beyond that, the Planck length means nothing. It's not the smallest length possible, it's just that we don't know how to describe anything smaller at the moment.

[UltraSane]

My understanding is that it is impossible to probe anything smaller than a Planck length because the required energy to probe such a small distance would create a black hole. So it is almost as if the universe conspires to prevent probing anything smaller.