The Red Mars series of books by Kim Stanley Robinson includes a range of measures that humans take to terraform the red planet, one of which is building robots to drill “moholes,” or holes that reach the Mohorovicic discontinuity between crust and mantle layers. Yes, they are giant volcanoes, and in the books the outgassing from these volcanoes is a strategy to increase the density and particularly the infrared absorption of the Martian atmosphere.