Johannesburg sits on the edge of the world's largest known gold deposit. The area was first declared open for public digging in 1886, although it is known that gold was traded and worked here for centuries before the arrival of gold prospectors. Within just a few years the city grew to be the largest settlement in South Africa and swelled with migrant workers from across Southern Africa and beyond. The so-called 'Randlords' (mining company owners) controlled much of the mining wealth and were responsible for some of Joburg's most recognisable buildings, which can still be admired today. Nowadays most mining activity in the Gauteng region is based in the west (gold and other minerals) and in the north (Cullinan diamond mine).