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31

Probably a bit over 4 km, in this South African mine: https://en.wikipedia.org/wiki/Mponeng_Gold_Mine But as the link mentions, the mine operators go to considerable lengths to reduce the mine temperature to endurable levels from the 66°C/151°F of the surrounding rock. Note: This answer is for the original question, where the OP asked for the deepest ...


19

Since you termed it based on sea level, the gold mines in South Africa are not the deepest, they begin at an elevation of ~1500 m, meaning their 4 km depth is only 2.5 km below sea level. The Kidd mine in Canada is 2.9 km deep and is located at an elevation of only ~250 m above sea level making it's depth 2.65 km below sea level. https://en.wikipedia.org/...


14

A cavern filled with natural gas would look like an ordinary cavern. However, natural gas is rarely abundant in caverns. Most natural gas reservoirs occur in the pore spaces of sandstones. These pore spaces are generally up to about 1 mm in diameter. Here's a photomicrograph of a sandstone — the green-blue colour is the pore space; Q, F, L, and M are quartz,...


5

It's just average ropy lava called pahoehoe by geologists. It's similar to dough so it easily forms ropes like the ones around the side and bulbs like the central area which is similar to a pillow except that it's called a toe. The colder black lava flows like ropes and the hotter red lava is smooth. This is also from Cleopatra probably: https://media....


4

According to 3-D Printing Artificial Reservoir Rocks to Test Their Petrophysical Properties Pore sizes of typical reservoir sandstone range from 0.1 to 100s of microns.


2

The speleothems are made of calcite, so breaking one small stalactite is same as breaking one narrow "straw" of calcite. Calcite is not so strong, so it is pretty easy to break small stalactite. Bone is much much stronger than one stalactite. Of course, the big ones (1 meter diameter) are harder to break. I see that you asked this here because you ...


1

It is extremely unlikely to be a sink hole. They are characteristic of waterlogged limestone country such as is found in Florida. Hollows are sometimes found in flows of igneous rock where there has been volcanic activity. Some hollows are large gas bubbles which were trapped when the rock around them solidified, but probably the most common hollows in ...


1

Deep caves primarily need two things: 1) Very thick or extensive tilted limestone beds so that there is enough bedrock to form a deep cave, and 2) A deep water table so that caves can form by dissolution of the limestone. Although phreatic caves form beneath the water table, they seldom form to great depth because the water becomes saturated with calcium ...


1

How are they formed? Erosion. Long version: The rock/cliff the cave is set in possesses different hard/softness. Some parts of it are more prone to erosion than others. Erosion comes in the form of waves and tides, washing and battering at the base of the cliff, nibbling away the soft parts of the rock quicker. Some chunks break off, rolling in the turf, ...


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