HANG Seng Index futures yesterday plummeted in high volume in the wake of the 171-point plunge on Wall Street last Friday. In index options, implied volatility soared to 35.75 per cent, up more than 15 percentage points to a level not seen since the heady trading days of the last quarter of 1993 and early 1994. March futures slumped 835 points or 7.5 per cent to 10,360, a discount of 37 points to the cash index. The close was the lowest on the contract this year. The contract was heavily marked down at the opening of trading, at 10,730, after the plunge in the Hang Seng London Reference Index on Friday. The contract fell throughout the morning. The low of the day was struck at the opening of afternoon trading, at 10,275. In the remainder of the afternoon session, the contract traded between 10,320 and 10,450. April futures also were down sharply. The contract fell 843 points or 7.5 per cent to 10,360. Volume was extremely heavy. Overall, there were 34,010 contracts traded, with 33,700 dealt in March and 310 in April. Open interest at the end of last week, before the Wall Street sell-off, was 49,146 contracts - 40,946 in March, 3,151 in April and 4,899 in June. Index option trading was also heavy. Investors were feasting on March puts all day. Put strikes at 10,200, 10,400 and 10,600 all saw more than 1,300 lots traded. The 10,000 put strike saw almost 500 lots traded. Call activity in March was mild by comparison, with activity in 10,800 and 11,000. Both strikes saw less than 500 lots traded. In April, activity focused on the calls with 10,400 through to 11,000 seeing action. None of these strikes saw more than 250 lots change hands. Investors today will take their cue from Wall Street trading overnight. Until yesterday's trading, large open positions in March had been accumulated. Investors intent on cutting losses or getting out while they were ahead probably partly accounted for the extremely heavy volume and heavy fall. Further declines today will trigger a further round of big margin calls which in itself could trigger more selling as investors clamber for cash.