Crude oil prices dropped sharply Tuesday evening, falling well under $100 per barrel after President Trump said the U.S. agreed to a two-week ceasefire with Iran that Pakistan had proposed.Why it matters: It's the biggest one-day free fall in oil prices since the 1991 Gulf War.Driving the news: The global benchmark Brent crude futures price fell roughly 16% to about $93 a barrel. But it's still far above the roughly $73 mark right before the war began at the end of February.WTI, the U.S. benchmark, was down about 19% to about $92 a barrel.The big picture: The agreement is subject to Iran to allowing "complete" reopening of the Strait of Hormuz, Trump said.The de facto closure of the waterway — which handles about a fourth of the world's seaborne oil trade — has brought the largest disruption in oil market history.What we're watching: Whether shippers have enough confidence in safe passage of the narrow waterway off Iran to resume shipments of oil, liquefied natural gas and other products.The physical oil market remains in turmoil after Middle East producers lowered output with export routes cut off.Dated Brent — the global benchmark for real-world, physical barrels of crude oil — reached its highest recorded price of $144.42, according to S&P Global Energy Platts. The previous record was set in 2008 at the height of the financial crisis.Iraq, Saudi Arabia, Kuwait, UAE, Qatar, and Bahrain collectively shut-in 7.5 million barrels per day of crude production in March, with a higher figure this month, according to Energy Department's stats arm.Editor's note: This story has been updated to reflect the historical context of the oil price plunge.