Oh, and Daedelus, thanks for that tip. I didn't know I could use diamond pipes to handle overflows so easily!!! It's certainly possible that was a contributing factor as I just checked and I filled 12 double chests with cobble. So, if they all started overflowing from my new larger quarry, with the extra water..... Anyway, my whole system is now undergoing a redesign and in the immediate interim, void pipes are installed and water is only being used as I uncover lava.