I took a taxi from Sucre to Potosí with Denise and Carla. We made it in record time, despite stopping to take photos of some amazing rainbows among the mountains along the way, because our driver was going at breakneck speed. We played some good music on the taxi ride - Carla has been educating us in reggaetón music which is very popular here, Llamada de Emergencia by Daddy Yankee has been playing on the radio and in clubs everywhere we've been in South America.

We arrived in Potosí at dinner time and I had the amazing Bolivian specialty dish 'pique macho', which consisted of beef, frankfurter sausages, chorizo and chips covered in onion gravy and topped with raw peppers, onions, chillis, boiled eggs, cheese and olives (wow!). This was at the great Café Pub 4,060, so-called because it is at 4,060 metres above sea level. Potosí is the highest city in the world, 'city' being the key word - Uyuni (where I'm going next) is the highest 'town' in the world and La Paz claims to be the second-highest capital after Tibet, even though it's not technically the capital city.

Potosí was distinctly more modern than Sucre. Apparently in the late 18th Century it was more wealthy and populous than London or Paris due to mining of the abundant silver in the mountains there - the Spanish brought millions of slaves from Africa to work there along with the indigenous South Americans; apparently eight million people died in the mines under Spanish rule. From 1800 the silver resources became depleted and the city went into decline. The mines are still being worked today, mainly for cheaper metals/minerals, but only by a few thousand people, and, sadly 500-1,000 of the workers are children (even though child labour is illegal in Bolivia, it is overlooked here).

Many of the mines are co-operatives, owned by the workers, who are paid based on the quantity of minerals they excavate rather than the number of hours worked and they buy all their own equipment and materials. A number of ex-miners have made a career out of taking visitors on tours of the working mines. I went on one such tour with Oochoo (a Quechua nickname, his real name was Juan Carlos) who had been a miner for three years when he was young but then had an accident and his wife refused to let him return to the mine.

I donned overalls, wellies, a hard hat and head torch and went to the 'miner's market', a street where the miners buy their wares. We bought some sticks of dynamite, soft drinks and coca leaves to give the workers. Then we visited an ore processing plant where the different minerals are separated into 'concentrate' that is then shipped abroad for refining.

Finally we went into the Candelaria mine. About 500 metres inside there is the beginnings of a small museum about the mines in Potosí, it includes the 'Tío' - every mine has a sculpture of this devil-like creature and all the miners venerate it, making offerings of coca, cigarettes and alcohol in the hope that it will appease him and save them from accidents and help them find minerals.

At this point visitors could decide whether or not they wanted to proceed deeper into the mine and see the lower levels where miners were working - I carried on... Further in it was hot and dusty and hard to breathe. The tunnels varied in size - sometimes I could stand up straight and other times I had to crawl. The walls and ceilings of the tunnels were lined with asbestos and arsenic.

In the lower levels we saw several men digging and moving the ore around in wagons before shovelling it into buckets that are winched up above ground. It is incredibly tough physical work in unpleasant and dangerous conditions. They work eight-hour shifts and cannot eat during that time so their cheeks are invariably stuffed full of coca leaves to relieve their appetite and tiredness. At the weekends many of the miners get drunk on a 96% sugar-cane alcohol to unwind.

As we climbed back up through the tunnels it was hard to breathe through my scarf so I had to keep taking it off, which meant I was breathing in all the dust and that made me feel a bit sick - and this was after only a couple of hours inside the mine - these guys do it eight hours a day, six days a week. Many of the miners contract lung diseases as a result of their work (most don't even wear dust masks), they have a life expectancy of just 40 years and even less if they start working there as a child - it's truly shocking. Most of the miners choose to do it because they can't get any other work and it pays relatively well; a few do it out of a sense of tradition but it was clear that most want a better life for their sons.

I wasn't feeling great after visiting the mine but a long nap and a shower sorted me out. In the evening we went back to Café Pub 4,060 and then in search of karaoke - we first found a place named Sebastian's which was truly weird and grotty - the walls were lined with broken mirrors, the ceiling covered in tin foil, the sink in the toilet full of vomit and the televisions didn't work so it wasn't even possible to do karaoke. We quickly left and found Nico's karaoke bar which looked more like a strip club but we were the only people there and it had a great big screen so we had a lot of fun belting out Hotel California, Billie Jean and ABBA classics Although some of the accompanying videos were a bit dodgy to say the least!