Mina Velha is one of Ouro Preto's old gold-mine visits, a modest but atmospheric counterpoint to the city's churches and steep colonial streets. The appeal is less spectacle than context: the tunnels help explain the labour, extraction, and risk behind the wealth that built the town above.
It is best treated as part of a wider mining itinerary rather than the single reason to visit. If you have already seen the grand Baroque centre, a mine visit gives the city a darker and more physical dimension, especially when paired with one of the larger mines around Ouro Preto.