Its gargantuan size - two Hurricane Andrews could fit inside it - spread so much fear that people all over the Florida Peninsula upended their lives to flee.
"This was a large, extremely dangerous catastrophic hurricane," National Hurricane Centre spokesman and meteorologist Dennis Feltgen said today, when he said the storm was over.
Colorado State University hurricane researcher Phil Klotzbach put it simpler: "Irma was a beast."
Irma generated as much accumulated energy in a dozen days as an entire six-month hurricane season would in an average year, Klotzbach calculated.
Just 30 hours after it became a tropical storm on August 31, Irma was a major Category three hurricane. By September 5 it had intensified into a Category four, with 210km/h winds, and it wasn't near done.
It became a Category five storm the next day with top winds of 300km/h, the highest ever recorded in the open Atlantic.
Only one storm whirled faster - Hurricane Allen reached 305km/h in 1980 over the normally warm Gulf of Mexico - but Irma held its ferociously high speeds for 37 hours, a new global record for tropical cyclones.
It beat Typhoon Haiyan, which also reached 300km/h before killing more than 6000 people in the Philippines. Irma ultimately spent 78 hours as a Category five, the longest in 85 years for Atlantic hurricanes.
Irma's entire path, from its birth off Africa to its death over the Ohio Valley, stayed within the cone of the probable track forecast by the National Hurricane Centre.
Irma claimed its first victim when it was still far off, sending a "monster wave" to drown a teenage surfer in Barbados. Then it hit the Leeward Islands in full fury, sweeping a 2-year-old boy to his death after tearing the roof from his home.
This photo shows the aftermath of Hurricane Irma in Big Pine Key, Florida.
BY THE NUMBERS - People still without electricity: 6.8 million, about a third of Florida's population, and hundreds of thousands in Georgia, with utilities saying it could take 10 days or more before all have power. - People still in shelters in Florida: 13,000. - Money raised by a star-studded "Hand in Hand" telethon for Harvey and Irma victims: US$44 million. Potential cost of damage to privately insured property in US and the Caribbean: US$55 billion.
View of the partially buildings destroyed by Irma in St Martin.
Irma bullied through much of the Caribbean - Antigua, St Martin, St Barts, Anguilla, the US and British Virgin Islands, Turks and Caicos, the Bahamas.
It narrowly skirted Puerto Rico, Haiti and the Dominican Republic. It turned lush tropical playgrounds into blasted-out landscapes, littered with splintered lumber, crumpled sheet metal and shattered lives. In St Martin, 15 people were killed.
Irma was still a Category five when it raked Cuba's coast, the first hurricane that size to hit the storm-prone island since 1924. At least 10 people died there, despite massive evacuations. And by moving briefly over land, it may have spared Florida a tougher punch.
More importantly, the system slowed, delaying its turn north and steering its centre over Florida's the west coast, which is less populated and less densely developed than the east.
It also allowed dry air and high winds from the southwest to flow into Irma, taking a bite out of the storm and even tearing the southwest eyewall apart for a while.
Irma was more vulnerable, but by no means weak. A Category four storm with 210km/h winds when it slammed into Cudjoe Key, it tied for history's seventh strongest hurricane to make US landfall, based on its central pressure. With Harvey's swamping of Texas, this is the first year two Category four storms hit the United States.
The Keys were devastated. Federal officials estimated that a quarter of the homes were destroyed, and hardly any escaped damage. Roofs seemed peeled off by can-openers; power poles were nowhere to be seen.
Irma was back over water as it closed in on mainland Florida, weakening still but spreading much wider - to more than 640km in girth - whipping the entire peninsula with winds of 60km/h or more.
It pushed its highest storm surge, 3m, onto Florida's southwestern coast, while causing some of its worst flooding in northeast Florida, Georgia and South Carolina, far from Irma's centre.
Irma's second US landfall was on Marco Island, near where Wilma hit in 2005. By then, Irma was a still-major Category three, with 185km/h winds, but weakening fast.
The worst of its fury somehow missed the Tampa Bay area, where homes were not nearly as flooded as those in faraway Jacksonville.
Irma then sloshed through Georgia and Alabama as a tropical storm, blowing down tall trees and power lines, before dissipating Tuesday over Tennessee and Ohio.
