One of the next stops along the way was the ancient and beautiful Roman town of Bath, in the south-west of England. It is famous for its geothermally heated hot-springs - the only naturally occurring hot springs in the UK, and is a world heritage site.

Andrews et al. report that the Bath springs originate from the Mendip Hills, which are limestone hills south of in Somerset. A maximum subsurface temperature of 8016 °C is found between 2.7 and 4.3 km underground within the limestone, and the water spends less than 10,000 years in the limestone. Atkinson and Davison report that the average surface temperature is between 44 and 47 °C.

Colours in hot springs are often caused by thermophilic microorganisms. These are organisms that, as the name suggest, love heat. They have provided such intrigue that there have been conferences devoted to the topic. Cyanobacteria, a common thermophile, grow in large colonies called bacterial mats that form the slime on the edges of the springs. Different coloured organisms prefer different temperatures, and so the colour of the bath can tell you its temperature - yellow is 70 °C, brown is 60 °C and green is 50 °C or lower.

The Bath springs were known to early settlers from about 7000 BP, and the legend of their discovery is attributed to the Celt Bladud who, having seen his pigs cured of leprosy in the mud that surrounds the springs, thought that the waters must have healing properties. The springs came to be associated with the Celtic goddess Sul, and when the Romans came, they were dedicated to the goddess Sulis Minerva.

The great Roman engineers were the first to harness the springs for their heat, with Bath becoming the first example of the use of geothermal energy for heat in Britain. It is only very recently that the locals and tourists have been able to do any more than touch the venerable waters with the opening of a brand new complex allowing you to bathe, as the Roman’s did, in the spring waters. At something like 50 pounds for 20 minutes and with my train due very soon after, unfortunately I missed out.