Underground most commonly refers to:
Underground may also refer to:
Underground is a 1941 war film about the German Nazi Resistance opposing the Nazis in World War II. Jeffrey Lynn and Philip Dorn play two brothers initially on opposite sides.
Underground is the second studio album by the American garage rock band, The Electric Prunes, and was released in 1967 on Reprise Records. It would be the final album of any materialized input by band members until the 1969 "New Improved" Electric Prunes were formed. The album was a moderate chart hit, but, without a hit-ready single, the band could not repeat their past success.
The album, Underground, brought changes to the band once recording began. Limited lyrical input plagued the band's creative process on their debut. Only one track, composed by Mark Tulin and James Lowe, called "Lovin'" was included on their first album. This changed with this album because Dave Hassinger, the group's producer, was not as active in the sessions resulting in nine of the 12 tracks becoming the band's own material. With so much more musical freedom, the band could mold their music into their own image. The final products were a more direct and cohesive set of songs that reflected the band's own design.
Geothermal gradient is the rate of increasing temperature with respect to increasing depth in the Earth's interior. Away from tectonic plate boundaries, it is about 25 °C per km of depth (1 °F per 70 feet of depth) near the surface in most of the world. Strictly speaking, geo-thermal necessarily refers to the Earth but the concept may be applied to other planets. A line tracing the gradient through the planetary body is called a geotherm on Earth and other terrestrial planets. On the Moon it is called a selenotherm.
The Earth's internal heat comes from a combination of residual heat from planetary accretion, heat produced through radioactive decay, and possibly heat from other sources. The major heat-producing isotopes in the Earth are potassium-40, uranium-238, uranium-235, and thorium-232. At the center of the planet, the temperature may be up to 7,000 K and the pressure could reach 360 GPa(3.6 million atm). Because much of the heat is provided by radioactive decay, scientists believe that early in Earth history, before isotopes with short half-lives had been depleted, Earth's heat production would have been much higher. Heat production was twice that of present-day at approximately 3 billion years ago, resulting in larger temperature gradients within the Earth, larger rates of mantle convection and plate tectonics, allowing the production of igneous rocks such as komatiites that are not formed anymore today.