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Urban Dangers
Tunnel boring in urban areas has to be carefully planned for a number of reasons. Mainly, urban areas are already laden with underground infrastructure, such as power, water, sewage, in addition to road plots and occupied real estate. Land subsidence is another, relatively minor consideration.
Human Risks
Humans working in unfinished machine-bored tunnels need to be fully protected against the hazards of an enclosed environment full of dust and mineral particulates, as well as lethal gas seams that are difficult to detect, and extreme heat and humidity. This danger is mitigated by limiting the amount of time humans can remain in unfinished tunnels.
Land Subsidence
Poor mapping, engineering or just plain bad luck can encourage land subsidence in an area being bored through, whether it's underground, through a hill or mountain, or underwater. Normally, tunnels are bored in such a way as to minimally disrupt the surrounding environment for safety reasons, but weaknesses can be inadvertently be exacerbated, causing tunnel collapses due to gravity.
Tectonic Activity
Disrupting solid rock with a very powerful boring machine can cause increased seismic, tectonic or volcanic activity, depending on the area. Tunnel boring is essentially the same as carving a manmade fissure in the earth, which can release plate or steam pressure, potentially causing earthquakes and landslides.