The 1720 bubble was the South Sea Bubble.
The South Sea Company received a monopoly over British trade with Spanish South America under the Treaty of Utrecht, although its practical trading opportunities were much more limited than many investors imagined. In 1720, the company promoted a plan to convert government debt into company shares, encouraging intense speculation.
Its share price rose rapidly as investors expected enormous profits from overseas commerce and debt arrangements. The boom attracted prominent investors and imitators, but confidence collapsed later in the year. The resulting crash ruined many people and triggered political scandal in Britain.
The South Sea Bubble occurred alongside John Law’s Mississippi Bubble in France, and the two are often discussed together as major early financial bubbles. The event was not a modern stock-exchange crash in the same sense as 1929 or 1987, but it is a landmark example of speculative finance, leverage, promotional claims, and crowd psychology.