I have some acquaintances in my city's water & sewer dept. When they dig a trench of 5 feet deep (1.5m) or more, they use some kind of shoring, trench box etc. At 20 feet deep, they have already reached for the phone and engaged an engineering firm with special equipment - they don't touch it themselves. People have lost their lives in collapses, so they take it seriously. Un-shored holes can be dug, but depending on soil type, the sides must be sloped, or sometimes stepped, to prevent collapse. Near vertical holes are only possible in bedrock. I am skeptical of accounts where the narrators take pains to mention "special" digging equipment, but fail to mention either shoring or sloping which such large holes would require. These narrators are either not good at estimating distances, or else not providing a very detailed account, or perhaps they have made up the figures to fit a 'narrative'. Giving only a length, width, depth and calculating a rectangular prism volume only leads to more questions, like did you measure the bottom of the pit, or the edges, what was the slope, or what system of shoring was employed?
I am also skeptical of the amount of explosives necessary to dig a 7m deep crater. As it happens, we have an example in Toulouse in 2001, 7m deep and 40m wide, resulting from 300 tonnes of ammonium nitrate. That's 661,000 pounds, roughly, about as much fertilizer as would fit in 3 American-railroad hopper cars.
https://en.wikipedia.org/wiki/Toulouse_ ... _explosion