Subsidence is a lowering or collapse of the ground (BGS, 2020). Uplift is the converse.

Tsunami is a Japanese term meaning wave (‘nami’) in a harbour (‘tsu’). It is a series of travelling waves of extremely long length and period. They are usually generated by seabed disturbances associated with earthquakes occurring below or near the ocean floor, but also by other mechanisms such as submarine landslides (IOC, 2019).

Subsidence is a lowering or collapse of the ground, caused by various factors, including groundwater lowering, sub-surface mining or tunnelling, consolidation, sinkholes, or changes in moisture content in expansive soils. Shrink-swell is the term applied to the behaviour of expansive soils, which are a group of soils that exhibit volumetric change in response to changes in moisture content, such that they shrink in response to desiccation and swell by hydration, resulting in ground subsidence and ground heave respectively (BGS, 2020).

A sinkhole is a closed depression in karst (a landscape resulting from the dissolution of soluble rock) by current or palaeo internal drainage, also known as a doline. This is one of several hazards that result in subsidence, i.e., lowering or collapse of the ground (adapted from USGS, no date; and BGS, no date).

Ground gases that result from material decay (natural or anthropogenic) typically include radon, methane, carbon dioxide, hydrogen sulphide, but may also include the break down products of other compounds, such as nitrogen, alcohols, alkanes, cycloalkanes and alkenes, aromatic hydrocarbons (monocyclic or polycyclic); esters and ethers, as well as halogenated compounds and organosulphur. Ground gases derived from magma (molten or semi-molten natural material derived from the melting of land or oceanic crust) include carbon dioxide, sulphur dioxide, hydrogen sulphide and hydrogen halides.