The main reason for going to Potosi is to do a tour of the Cerro Ricco mines - "The mountain which eats men".
For those interested in the mines then the movie The Devil's Miner is well worth watching.
I watched it at hostel, which at times was straight out of Faulty Towers. I had to buy a ticket from reception to watch the film, only to have the receptionist come and collect the tickets from you before he started the DVD. Nice enough hostel, but odd in places.
There are many different groups, co-ops, working different part of the mountain, some in small groups (3-6) and some in larger groups (high teens), depending on the type of mining they are doing (targeted, smaller scale, mining or larger-scale mass extraction).
This is something you have to do an organised tour for, where I chose Koala Tours based on their reputation. The other sensible option would be Big Deal, which is owned by the miners themselves - where one of the people I travelled with across the Salt Flats and to San Pedro with used them and gave good reviews. Either are worth considering and having a good tour operator is worth spending the shoe leather and cash on.
With our tour we were split in to groups of 6, with each group having 2 guides - one at each end of the group, providing a higher degree of safety and allowing someone to leave the group if needed (none in our group did, although one very tall chap (easily 6'6", if not more) from another group didn't make it - I'm surprised the tour company said he should have even considered it, in my absolutely inexpert guess he is just too tall - anyone over about 6' is going to struggle).
If you are considering going and are: 1) (very) tall; 2) have back problems 3) have breathing issues; 4) claustrophobic; 5) not physically strong enough to man-handle yourself around in small spaces - think twice; this isn't a theme park and there is no emergency exit - you have to get yourself in and out, dealing with whatever you find. I would recommend going, just think twice about if you can actually do it.
To visit the "face" you have to walk, stoop and in places crawl on all fours to get through the small, cramped, tunnels, and then climb down (and later, back up) rocks to get from the 1st (entrance) to the 3rd level, a decent of 60m according to the guide, which it didn't feel like, all whilst in a hot and dusty environment, having entered the mine at about 4200m.
I can't imagine having to do that twice a day, one of them after 12+ hours of back-breaking manual labour, drilling, smashing, blasting and sorting the rocks to collect the ore (mostly silver, but other minerals are also extracted), all for very little money (one miner said that on a good day he'd earn £10 for his work, but it isn't always good days and there are no fixed wages - it is all about how much ore they extract, its quality (percentage of ore to everything else) and the world silver/mineral markets).
Side note: I actually enjoyed the climb back up - added "climbing" to my list of things to try when I get home.
The "face" we visited was a small cave like space which had been blasted out, with three miners in there smashing and sorting the various rocks - trying to separate out the better, higher concentration of ore, rocks from the bulk - the better rocks being kept separately to the bulk, which was pushed down another blasted hole in the rock to a waiting cart. It wasn't clear, but I think all is sold and the "better" rocks get a better price, thus kept separately from the bulk.
Guys move the mined material around by hand - three guys move a 1 ton cart (2 push it, 1 pulls it) - and once that cart is moving everyone else has to get out of the way of it; there are no brakes and it will not stop for you. Our guides were very good at keeping an ear open for carts and ensuring we had moved to (and at times, ran to) a suitable passing place.
To say that this is a hard job is an understatement.