1) Bad news on the stock market 2) Big drop 3) Calamitous decline 4) Disastrous drop 5) Dive 6) Dramatic decline 7) Plummet 8) Plunge 9) Plunge nose first 10) Plunk 11) Precipitous plummet 12) Quick fall 13) Quickly fall 14) Rapid decline 15) Sharp fall 16) Sudden drop 17) Sudden plunge 18) To dive down in a steep angle 1) Tailspinach 
A sudden drop or plunge in prices, values, etc
[v] - plunge nose first
noun a sudden sharp drop or rapid decline; `the stock took a nosedive`