>>2225731>(a) Flooding. Build above the waterline, dig sumps for rainwater and optionally use coatings of some kind on the tunnel ceiling/walls to discourage water infilatrating
>(b) Dangerous gas buildup. Number one in my mind. All tunnels should be designed with ventilation as a primary consideration for this reason. Solar chimneys to create passive circulation, windscoops, manual pump backups, emergency perchlorate candles, all important. If defensibility is a concern, installation of grease traps or other methods of diverting gas grenades from ventilation tubes to the surface would need to be considered. Similarly, being able to seal off any given ventilation shaft/tube to prevent gas being pumped down (see: the mighty mite)
>(c) Electricity. Depends so much on what needs to be accomplished. Electricity for a small group could be as simple as small solar panels in concealed spots during the day, charging battery banks to be collected at night. Could also be as elaborate as giant stirling engines running off the thermal variation between soil and air temperatures. Lots of typical uses of electricity (lighting, cooking, heating) could conceivably be met without using any.
>(d) Easily disoriented. True. Maybe coded markers/signs, or intentional variations in tunnel geometry could help? "The walls are rough here, so I'm definitely in the eastern tunnel. Smooth walls in the western one." sort of thing.
