Features seem generated after structures during world generation.

This can cause issue with monster_rooms, mineshafts, or even villages when it occurs near from the surface (with ice spike covering some ice village houses for example)

This attached screenshot show a custom configured structure features, using the village type, that generates an undeground room. Each structure of the template_pool are called with the single_pool_element element_type. The geode pierce the soil, replacing both solid and air blocks.