Mineral Water

The water that fills the Great Bath is around 10,000 years old. It fell as rain water on the Mendip Hills, 15 miles to the south of Bath, when Mesolithic people were using the natural hot spring, bubbling out of a woodland area in 7500 BC.

The water flows underground from the hills along a fault line called the ‘Penny Quick Fault’ and collects in an underground lake, 2 miles down. The water in the lake gets heated to around 90 degrees C by the earth's core. A tremendous amount of pressure builds up in the lake forcing the water up through a large fissure in the rock allowing it to bubble up to the surface.

The Romans built a reservoir to contain this hot water. By the time the water has travelled the 2 miles up to the surface it has cooled down to 46 degrees C, that’s still about 10 degrees C hotter than a comfortable bath or shower.

After the water had collected in the reservoir the water would have been directed to a number of pools. Today the water only flows into the Great Bath or out of the Great Drain down to the River Avon. Thirteen litres of water flow into the Great Bath every second. This means that you could fill your bath at home in approximately 6 seconds! The temperature of the water in the Great Bath is 36 degrees C, just the right temperature for a bath.

Point where the water flows into the Great Bath

As well as being hot, the water picks up 43 kinds of metals and minerals in the ground. The largest concentrations of minerals are calcium and sulphate. The water is low in dissolved metals except for iron, which causes an orange staining around many parts of the pools.

Iron staining on the inflow channel to the Great Bath

If you have seen the Great Bath you can’t help but notice that the water is a lovely green colour today. When the water comes up from the ground it is colourless, the green hue is from the algae growing on the surface of the water, caused by its heat and daylight. When the Romans were using the Great Bath it was covered by a roof, eliminating direct sunlight, this stopped algae from growing. Unlike today, you could have seen the bottom of the Great Bath.

The green water of the Great Bath

Posted by Laura (Visitor Services Assistant)