It usually means soft rock. It could be a void or pocket like...

It usually means soft rock. It could be a void or pocket like you mentioned but i don't think so. You can here one of the guys say something like "That's where all our mud is going". They are referring to their drilling fluids or "muds". In rock formations that are not porous the water that you pump down gets returned out the top of the hole. The fact they are not getting water return means they have hit a porous layer - Given the geology of the area and the hole depth - its probably an aquifer. Lets hope it has brine!