Publicly disclosed boring/tunnel machines put up interlocking curved concrete panels up as they progress (see Chunnel Project documentary). Rumored black project boring machines reportedly melt the rock and reposition/compress it to the sides creating an airtight lava-tube type of tunnel as they go. These supposedly exist at ~10km depth under all landmasses (speculation).
Publicly disclosed boring/tunnel machines put up interlocking curved concrete panels up as they progress (see Chunnel Project documentary). Rumored black project boring machines reportedly melt the rock and reposition/compress it to the sides creating an airtight lava-tube type of tunnel as they go. These supposedly exist at ~10km depth under all landmasses (speculation).