Anticline photo courtesy of Cleet Carlton of Golden Gate Photo (fair use policy)
An anticline is a fold structure in which the sides of the fold slope apart. Its shape is concave downward. Technically, it is a fold with younger rocks on the outside, so technically this should be called an antiform until we can confirm the relative ages of the rocks.
Deep underground, anticlines are highly desired by petroleum prospectors. The rock beds trap rising hydrocarbon fluids, allowing oil and gas to build up in the pore space of the anticline rocks. The largest oilfields occur in large, gentle anticlines in thick sedimentary rock sequences.