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Ultramafic rocks
An igneous rock consisting almost entirely of iron- and magnesium-rich minerals with a silica content typically less than 45%.

UNCLOS
United Nations Convention on the Law of the Sea, which was adopted at Montego Bay on December 10, 1982.

Unconformity
A geological surface separating older from younger rocks and representing a gap in the geological record.

Under-saturated
A solution that could contain more solute than is presently dissolved in it.

UNFCCC
United Nations Framework Convention on Climate Change, which was adopted at New York on May 9, 1992.

Unminable
Extremely unlikely to be mined under current or foreseeable economic conditions.

Updip
Inclining upward following a structural contour of strata.

Upper ocean
The ocean above 1,000m depth.