Subsurface transportation refers to craft for traveling beneath the surface of a planetary or stellar body or habitat. It mostly includes hardened probes for exploratory work in planetary or stellar interiors, although automated drilling machines for the creation of vac-trains and other forms of subsurface transit are also included in this category. In rare cases sentient subsurface vehicles have also been created, although there is debate as to the difference between these and automated probes since often the such craft use upload or ai crews.
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Vac Trains - Text by Todd Drashner Fast train transport in evacuated tubes to reduce atmospheric resistance.