Did You Know?
The Longest Commute on This List Is Ninety Minutes and Goes Straight Down
West of Johannesburg, in the Witwatersrand Basin, is the Mponeng mine. It is the deepest mine on Earth, running more than four kilometres below the surface.
About 4,000 miners descend every day. They ride in multi-stage cages carrying up to 120 people at a time, at speeds around 60 km/h, and the journey from daylight to the working face takes over an hour — some accounts put it at ninety minutes. That is a commute measured in vertical kilometres.
Conditions down there are governed by the local geothermal gradient, which is unusually gentle: roughly 10°C hotter per kilometre of depth. Rock walls reach 60°C and above, and humidity often exceeds 95%. To make it survivable, the mine pumps more than 6,000 tonnes of ice slurry into underground reservoirs and runs enormous fans.
Each day the operation uses around 2,300 kg of explosives and removes some 6,400 tonnes of rock. Some working spaces are 60 centimetres wide. The gold-bearing reef they are chasing is 2.7 billion years old.
Johannesburg itself has no river, no harbour and no lake. It is one of the very few major cities on Earth with no natural reason to exist. It is there because in 1886 somebody found gold, and within a decade there was a city.
The clock: above ground, Johannesburg keeps unusually honest time. South Africa runs on UTC+2, built for the 30th meridian, and Johannesburg sits at 28° east — meaning its clock is within about eight minutes of its own sun. On a list that includes cities running an hour ahead of theirs, that is close to perfect.
Below ground, something stranger is true. Clocks deeper inside a gravitational field genuinely run slower than clocks above them — this is ordinary general relativity, and modern atomic clocks can measure the difference across height changes of a few tens of centimetres.
So four kilometres down, at the bottom of Mponeng, time is passing very slightly more slowly than it is at the surface. The effect is far too small for any miner to notice, and it is not a metaphor. It is simply the case.
Johannesburg may be the only city on this list where the lift is, technically, a time machine.
Sources
- Guinness World Records, *Deepest mine* (over 4 km; 6,000 tons of ice slurry; ~2,300 kg of explosives and 6,400 tons of rock per day)
- Wikipedia, *Mponeng Gold Mine* (more than an hour's journey from the surface to the deepest point)
- IFLScience and Mining.com (4,000 miners; 60°C rock temperature; 95% humidity)
- Harmony Gold official page
- Longitude calculation: Johannesburg 28.05°E, UTC+2 meridian 30°E → ~8 minutes