Which Hong Kong stock index plunged during the 1997 Asian financial crisis?

The Hang Seng Index plunged during the 1997 Asian financial crisis.

Hong Kong's benchmark index suffered a particularly severe sell-off in October 1997. On October 28, it fell 1,211.47 points, or 6.7%, as investors worried about regional currency turmoil, weakening economies, and the sustainability of Hong Kong property and equity valuations.

The Hang Seng Index tracks major companies listed in Hong Kong and is one of Asia's best-known market indicators. Its movement reflected both local conditions and international concern about contagion from Thailand and other Asian economies.

The crisis began with pressure on Thailand's baht after the currency was allowed to float in July 1997, then spread through several economies. The Hang Seng was not a currency: it was a stock-market index. That distinction separates this answer from questions about the currencies affected by the crisis.