When a geologist looks at Google Maps images, we usually filter out any human activity. But in the case of mines, that would be a mistake – the holes humans dig can tell us about the geology.

What’s this? A big circular hole in the ground and large piles of bluish mine waste. The shadows are on the northern side, showing we are in the Southern hemisphere. This is the Letlhakane mine in Botswana.

And here’s another one, in the northern hemisphere. The hole here is half a kilometre deep (it’s an older mine, the Mirny mine in Siberia. They used jet engines to melt the soil so they could dig).

And again, the Diavik mine in Canada. Note that the hole is considerably deeper than the lake it sits in. They really wanted to dig that hole and yet most of the its contents are sitting in those big blue piles.

Finally the oldest example in the world, the Big Hole in Kimberley. The piles of dirt were put back in the hole, or covered in houses. Mining underneath here reached a kilometre depth.

We’ve been looking at some famous diamond mines. Diamonds form deep in the earth and those worth digging holes for only reach the surface via weird fizzy molten rock called kimberlite. This magma zips up from 100km depth to the surface in only hours. Travelling at depth along a narrow crack (or dyke in geo-speak ) when the magma reaches the surface it forms a carrot-shaped pipe. The magma solidifies, peppered with diamonds that formed at depth and were pulled up inside it.

The pipe is circular in cross-section, so as the miners dig out the kimberlite they leave a circular hole. The vast majority of what is dug out is waste – only the precious diamonds are extracted. Kimberlite is bluish in colour, as you can see from the piles of it above.

The Big Hole in Kimberley was the first kimberlite pipe ever identified , in the 19th Century. Diamonds found before then were from placer deposits, river gravels that contain diamonds eroded out of kimberlite pipes. Diamond placer deposits were first discovered in India and then Brazil. But the biggest area for modern mining is on Namibia’s Skeleton Coast.

Southern Africa’s Orange river rises on the Kapvaal Craton, an area rich in kimberlite pipes. For 100 million years it has flowed across the continent into the Atlantic ocean, leaving thick placer deposits. These have since been pushed around since by wind and ocean waves to cover a wide area.

Starting at the beginning of the Twentieth Century, German settlers found diamonds and the government designated a huge area of land as Sperrgebiet – forbidden territory. This whole area remains closed, but active mining is concentrated the southern end, on the coast, where the diamonds are concentrated in ancient beach sands.

The mine here looks very different from the others, no round hole or blue kimberlite (but look for the regular patterns on the spoil heaps). Like the other mines, this one was caused by the desire for the beauty and strength of diamonds.