An "idiom" or "figure of speech".
just a sink Below surface (grade) sink
Easy money.
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I would call this convection currents in the mantle.
An intrusive igneous rock.
An intrusive igneous rock.
Molten material below the surface of the Earth is called magma. It consists of molten rock, gases, and suspended solids. Magma can eventually erupt onto the Earth's surface as lava through volcanic activity.
A submarine is a ship that can travel underwater.
Depending on which surface you want to name, perhaps the table -- the top of the cut stone, or a facet. There are many names for the facets. Learn more, below.
That would be a snorkel.
A French leader coined the phrase. See better answer on link below.