[The] Clemmensen reduction is a chemical reaction described as a reduction of ketones (or aldehydes) to alkanes using zinc amalgam and concentrated hydrochloric acid .

Source : Wikipedia

The reaction, which is used for converting a carbonyl group into a methylene group, was developed in the early 20th century.by the Danish chemist Erik Christian Clemmensen.

The replacement of the oxygen atoms of the carbonyl group in an aldehyde or ketone by two hydrogen atoms through the use of amalgamated zinc and hydrochloric acid was first employed in 1913 by Clemmensen and is know as the Clemmensen method of reduction. The process has been applied to a large number of aldehydes and ketones as a step in the synthesis of polynuclear hydrocarbons and alkylated aromatic compounds. It has also played an important role in the elucidation of the structures of highly complex natural products.

Source : Organic Reactions [reference book]

Despite a century or so of widespread use in the laboratory and in industry, an exact description of the chemical mechanisms of the reaction is still lacking.