The 1720 British speculative mania was the South Sea Bubble.
The South Sea Company received a government-backed monopoly over trade with Spanish South America in 1711, although its actual commercial opportunities were limited. In 1720, Parliament approved a plan for the company to assume part of Britain’s national debt, encouraging investors to buy its shares.
Promotional enthusiasm drove the share price dramatically higher during the year. The boom spread into other ventures, including many dubious companies formed to exploit public excitement. When confidence weakened, the South Sea Company’s shares fell sharply, ruining many investors and provoking a political scandal.
The South Sea Bubble is often discussed alongside France’s Mississippi Bubble, which also collapsed in 1720. It was not a modern exchange crash in the technical sense, but a major early example of debt-financed speculation, promotional excess, and a contagious loss of confidence.