fall dramatically — to decrease or decline very quickly and significantly

a shocked trader watches as red arrow lines plunge downward on multiple screens around him. His face shows complete dismay as he points at the rapidly descending chart lines. The graph lines were high just moments before but now crash toward the bottom of the screens. Numbers cascade downward in a steep diagonal pattern across all the monitors. Other traders in the background throw their hands up in panic and disbelief. Some cover their faces while others frantically gesture at their own screens. values falling dramatically in a very quick and significant decline