Underworld solves the equations of slow, viscous flow on a finite element mesh while carrying material history on particles that move through it. That combination is what lets a model track stress, damage and composition through the very large deformations that geology produces — mountain building, subduction, mantle convection over hundreds of millions of years.

The current version, Underworld 3, is a ground-up rewrite. Models are written as symbolic expressions in SymPy, which the code compiles to C and hands to PETSc to solve. You write the physics; the machinery for discretising, differentiating and solving it is generated. There is a note on how that works.

Underworld is open source, developed in the open, and supported by AuScope under the Australian Government’s NCRIS programme.