From deep in the well a mixture of oil and water is pumped into these separating tanks. The clever system works by gravity. The oil is lighter and floats to the surface where it is skimmed off and sent to the holding tank. The water leaves the tank from the bottom and goes to the disposal well. In the early days, storing oil proved difficult. Wooden tanks could catch fire and iron tanks could draw lightning. The pioneer men learned to store oil underground to keep it cool and safe from fire. They were able to create vast underground holding tanks because the clay soil of Lambton County was impervious. Using spades, the men dug tanks 18 metres deep and 9 metres across that would hold up to 280,000 Imperial gallons (8,000 barrels).