The Hunts and allied investors accumulated enormous physical and futures positions through 1979, at one point controlling a large share of the deliverable supply. Silver rose more than eightfold. In January 1980 COMEX adopted Rule 7, limiting positions and putting silver into liquidation-only, while margins were repeatedly raised.
With no new buying permitted the price broke. On 27 March 1980 — Silver Thursday — the Hunts failed to meet a $100 million margin call and the resulting forced selling took silver down about 50% in a single day, threatening their brokers' solvency and requiring a bank consortium loan.
Almost every modern rule the futures market runs on — position-limits, accountability-level, aggressive margin-increase powers — traces part of its lineage to this episode.
Example: a position of 100 million ounces equates to 20,000 COMEX contracts. A $1 fall in silver is $5,000 per contract, so the move from $50 to $11 implied roughly $3.9 billion of losses on that notional.
Related: silver-futures, position-limits, margin-increase, liquidation-only, market-manipulation