Irma's rampage across Florida broke records in Jacksonville before heading north to torment Georgia and beyond.
Jacksonville, the largest city geographically in the country, is grappling with a record storm surge and immense flooding. The city's Memorial Park has turned into an unrecognizable lake.
"We have very serious, significant river flooding along the banks of the St. Johns River. It's bad now, it's going to continue to get worse," meteorologist Angie Enyedi said. "We've already surpassed historic levels, the levels will continue to rise."
The storm was still hurling 65 mph winds Monday, pummeling cities in northeast Florida that had not expected to feel its full wrath.
Emergency workers in Daytona Beach rescued 25 people with a high-water truck after they were suddenly caught in an onslaught of wind and rain.
"It wasn't supposed to be like this," CNN correspondent Sara Sidner said from Daytona Beach. "It's been strong enough to knock us over."
Another 125 emergency rescues were made in less than an hour Monday in Orange County, home to Orlando -- an inland city to which many coastal residents had evacuated before the hurricane.
Now the entire Georgia coast is under a storm surge warning as Irma continues its destructive march north.
Extent of devastation not entirely clear
Irma made landfall on the Florida Keys as a Category 4 hurricane Sunday, but the full extent of the damage there is still unknown.
That's because some islands are extremely difficult to access. Southbound US Route 1 -- the only road connecting the Keys -- is closed, Florida Keys spokesman Andy Newman said.
Large debris is blocking access, and a 150-foot stretch "has some buckling," Newman said.
On top of that, "there's no electricity throughout the Keys. No cell service in at least the lower and middle Keys," said Bill South of the National Oceanic and Atmospheric Administration.
But there was some good news: There are no known fatalities on the Keys, Newman said.
The latest developments:
-- More than 6.2 million electric customers are without power in Florida, Gov. Rick Scott's office said Monday. FEMA chief Brock Long has said some places won't have electricity for weeks.
-- Irma continues to weaken as it moves over northern Florida, the National Hurricane Center said. As of 11 a.m. ET, Irma packed 65 mph winds and was moving west-northwest at 17 mph.
-- Storm surge warnings in Florida, Georgia and South Carolina mean "there is a danger of life-threatening inundation," the hurricane center said.
-- More than 17,000 customers have already lost power in Savannah, Georgia.
-- In Venice, Florida, the water plant was shut down after it was damaged by the storm.
-- Miami streets turned into raging rivers, and the city's airport is closed because of significant water damage.
'It's the worst storm I've ever seen'
Irma is plowing into Georgia and toward other parts of the Deep South -- Alabama, Tennessee and the Carolinas. But that doesn't mean all Floridians should try to go back home.
"We're asking folks to be patient and remain sheltered in place," said St. Augustine Fire Chief Carlos Aviles.
"Stay off the roads, stay off the streets, let us complete our assessment, clear the roads of water, power lines, trees and then you can get out there and determine what happened to your individual property or your neighborhood," said Fort Lauderdale Mayor Jack Seiler.
The massive storm triggered evacuation orders for 5.6 million people before it made two landfalls in the state Sunday.
The first was over the Florida Keys, and the second, over Marco Island, left the island without water and power, authorities said.
"It's the worst storm I've ever seen," said South, the NOAA meteorologist.
More states brace for Irma
Georgia Gov. Nathan Deal has already declared a state of emergency for all 159 counties as Irma barrels toward the state. Cities as far inland as Atlanta are under a tropical storm watch Monday and Tuesday.
North Carolina Gov. Roy Cooper also declared a state of emergency for all 100 counties in his state ahead of Irma.
South Carolina Gov. Henry McMaster issued a mandatory evacuation for some barrier islands.
And in Alabama, some city school districts including Birmingham, Huntsville and Auburn planned to close Monday and in some cases Tuesday.
Irma's deadly trail of destruction
Before slamming into to the United States, Irma hit Cuba late Friday as a Category 5 hurricane.
It killed at least 26 people before leaving the Caribbean for Florida.
This is the first year on record that the continental United States has had two Category 4 hurricane landfalls in the same year.
Last month, Hurricane Harvey devastated much of coastal Texas and killed more than 70 people.