On June 17th 1972 five men were caught breaking into the headquarters of the Democratic National Committee in the Watergate Hotel in Washington DC. They were planting listening devices.
The Nixon administration attempted a cover-up, but details eventually became public, including audio tapes of conversations about the break-in. A number of people received jail sentences for their part in the scandal, and President Nixon, rather than face impeachment, resigned on August 9th 1974. He was the only US President ever to resign from office.
Watergate made a permanent contribution to popular culture - the practice of adding the suffix "-gate" when describing later scandals. For example, the Iran-Contra scandal was sometimes referred to as "Iran-gate".