Looks good to me! Perhaps the bottom of the loaf gets hotter faster and the yeast dies off sooner...
in the lower half, but more likely those holes were created while the bread was rising. If you let the dough rise once in a bowl, then just turned it out onto a baking sheet without shaping it, then the yeast didn't get distributed very well. Usually rolling dough into a loaf shape will do the trick, but the charm of ciabatta is that it is just dumped out freeform.
Try this: once you see that the dough has started to rise, after an hour or so, turn it out on your work surface, pat it out, and fold it in thirds. Pat it out again and fold it again. Put it back in the bowl to rise all the way. This redistributes the yeast and makes for a stronger rise. You don't really lose a whole hour because it bounces right back.