A gradual sinking to a lower level WordNet 3.0 & Open English WordNet

Settling is the process by which particulates move towards the bottom of a liquid and form a sediment. Wikipedia

A chemical process Wikipedia Disambiguation

the process where particulates settle to the bottom of a liquid Wikidata

The sudden sinking or gradual downward settling of the Earth's surface with little or no horizontal motion. The movement is not restricted in rate, magnitude, or area involved. Subsidence may be caused by natural geologic processes, such as solution, thawing, compaction, slow crustal warping, or withdrawal of fluid lava from beneath a solid crust; or by man's activity, such as subsurface mining or the pumping of oil or ground water. OmegaWiki

The act by which something subsides. Wiktionary