What caused the crisis at Chiquita and Sunshine Canyon?

Chiquita Canyon: A massive subsurface chemical reaction (effectively an underground fire) has tripled in size, releasing hazardous fumes and causing toxic liquid waste ("leachate") to bubble to the surface.

Sunshine Canyon: Heavy rainfall in late 2022/2023 soaked decomposing waste, creating an environment for bacterial growth and unchecked chemical reactions that have made odors uncontrollable.