Imperial gallon (gal)
Definition: An imperial gallon is a British unit of volume equal to exactly 4.54609 liters.
History/origin: The imperial gallon was standardized in the United Kingdom in 1824 as part of the imperial measurement system.
Current use: Imperial gallons still appear in some UK and Commonwealth contexts, especially historical references, fuel economy, and certain product descriptions.
Cubic kilometer (km³)
Definition: The cubic kilometer is a huge metric volume unit. A 1 km by 1 km by 1 km cube contains 1,000,000,000 m³, or 1,000,000,000,000 L.
History/origin: Earth science needed a readable unit for massive natural volumes, so km³ became useful for water bodies, ice, rock, magma, and atmosphere-scale quantities.
Current use: Km³ appears in hydrology, climate reports, ocean volume estimates, glacier studies, volcanic eruption data, reservoir capacity, and large-scale geography.