Catastrophic and life-threatening floods are still drenching Houston, Texas, America’s fourth-largest metro area.

Around four feet of rain has fallen in the city. Harvey has broken the national record for rain from a single tropical storm. Roadways are now rivers. Thousands have been rescued from flooding homes, and search efforts are still underway. A total of 12,000 National Guard members have been called into service, and civilians — from in and outside of Texas — have volunteered their boats for search and rescue efforts. More than 30,000 people will need temporary shelter, FEMA announced Monday, and estimated that 450,000 people are likely to seek federal disaster aid. Cleanup and rebuilding is likely to take years. At least 11 people have died in the storm, including a police officer who drowned in a patrol car, authorities report.

How did this situation get so bad?

To help answer, I called up Phil Bedient, an engineer who directs a research group at Rice University devoted to studying Houston’s flood, hurricane, and natural disaster preparedness, and Hal Needham, a geographer and coastal flooding expert who runs a consultancy business in Galveston, Texas.

They walked me through the dynamics of what made this a particularly drenching storm, why Houston is so susceptible to flooding, and whether the decision not to evacuate the city was the right call.

1) Why was the storm itself so bad?

Hurricane Harvey landed off the Gulf coast near Rockport, Texas, Friday night as a Category 4 storm with 130 mph winds. Since then, the storm has barely moved, even as its winds downgraded to tropical storm strength. Why? A few reasons.

Upper-level winds in the atmosphere usually steer big hurricanes and keep them moving after they make landfall. With Harvey, those steering winds broke down, and a high pressure system to the northwest kept Harvey locked in place.

That’s why Harvey is such a catastrophic rainfall disaster. It just keeps dumping heaps of rain over a very wide area around Houston. Forty-two inches of rain — nearly 4 feet — have been recorded in some areas. “To get that much rain, over a wide area, and have the coastal areas elevated with storm surge — the waters are going to struggle to drain,” Needham said.

UNBELIEVABLE RAIN TOTALS: 43" South Houston, all areas in white (almost 140 miles wide) have reported more than 20" and up to 43"! Not over. pic.twitter.com/DHbhLa2Iad — Ginger Zee (@Ginger_Zee) August 29, 2017

Here’s another way to think about the scale of rain dumped by the remnants of Hurricane Harvey. According to meteorologist Ryan Maue at WeatherBell, as of Monday afternoon, around 14 trillion to 15 trillion gallons of water had fallen on Houston and its surrounding areas. (The calculation is simple, he says on Twitter: It’s depth of rain multiplied by the number of square miles covered.) And 5 trillion more gallons are still expected to come.

Climate change is likely part of this flooding story too, as Vox’s David Roberts explains. Sea-level rise in the Gulf, increased summer heat, and increased water temperature all likely conspired to make this storm slightly worse.

2) Why is Houston particularly prone to flooding?

The rains from Harvey are exceptional — some areas saw 15 inches in 24 hours, a record for the city. “No major urban area can survive that [amount of rain] without a problem, without some flooding,” Bedient says. But Houston is particularly susceptible for a few reasons, he explains.

A) The city is flat. “Like a pancake,” Bedient says, which make drainage hard. “The water has nowhere to go.”

B) Urban sprawl — meaning low density development over a large area — has limited the city’s natural drainage capacity. The reasoning is pretty simple: When concrete is poured over green space, the city loses capacity to absorb water. Sprawl has been exacerbated by a recent population boom and development growth in the city. But the problem goes back decades.

“We’ve had a lot of early-on development in the ‘60, ‘70s, ‘80s that created this rampant urban expansion,” he says. “The building practices were the Wild West. Drainage didn’t keep up. ... It’s a developer-run community and city.” This means concrete has gotten priority over maintaining green space, where vegetation and soil can help soak up the water like a sponge.

In the 1940s, Bedient says, engineers built reservoirs on the west side of Houston to protect the downtown area from flooding. But since, those reservoirs have become over-taxed. “Normally when you build big reservoirs like that, you’re also going to try to protect what flows into those reservoirs,” he says. “Well, that upper end has become rampantly developed that feeds excess water into the reservoirs, more water than they were ever designed to handle.”

C) Bedient says there’s been “the lack of a will or a policy to really get serious about flood control.” ProPublica documented this problem in a 2016 investigation:

As millions have flocked to the metropolitan area in recent decades, local officials have largely snubbed stricter building regulations, allowing developers to pave over crucial acres of prairie land that once absorbed huge amounts of rainwater. That has led to an excess of floodwater during storms that chokes the city’s vast bayou network, drainage systems and two huge federally owned reservoirs, endangering many nearby homes...

Scares and near-misses like Hurricane Ike in 2008 prompted discussions about building additional coastal flooding protections, like dykes and levees, but these didn’t go anywhere.

Harvey is the third 500-year flood in to occur in Houston three years. “Obviously there’s something wrong,” says Bedient. He hopes the flooding from Harvey will serve as a wake-up call. “This is Houston’s Katrina, if you will,” he says.

(One note: 500-year flood is a term that describes the probability of a flood event occurring in any given year, not its actual frequency. Vox’s Dara Lind has a great explainer on the term.)

3) Should Houston have been evacuated?

When the National Weather Service was advising Houston residents to get on their roofs in the case the top floor of their home became unsafe, it’s easy to think: Why didn’t those people evacuate?

For one, there was no evacuation order issued before the storm hit (some Houston communities have since been evacuated).

At a Sunday press conference, according to the Associated Press, Houston Mayor Sylvester Turner defended the position. “If you think the situation right now is bad and you give an order to evacuate, you are creating a nightmare,” Turner said. This “nightmare” comes from experience. In 2005 Houston issued an evacuation order for 2.5 million people for Hurricane Rita. It was chaotic: More than 100 people died while evacuating. People were stuck on the roadways for days.

Related The best places to donate for Hurricane Harvey disaster relief

Plus, Harvey struck with only a few days’ notice, and an uncertain forecast. An evacuation may have been especially dangerous considering how some Houston freeways are actually designed with features to help drain Houston floods. Ian Bogost explains in the Atlantic:

To account for the certainty of flooding, Houston has built drainage channels, sewers, outfalls, on- and off-road ditches, and detention ponds to hold or move water away from local areas. When they fill, the roadways provide overrun. The dramatic images from Houston that show wide, interstate freeways transformed into rivers look like the cause of the disaster, but they are also its solution, if not an ideal one.

Imagine if people were stuck in traffic when the flood waters started to rise. That’s a disaster in and of itself. “You cannot evacuate 2 million people out of a metropolitan area you can’t do it,” Bedient says. “You can’t do it anywhere.”

Needham agrees. “Evacuating all of Houston is not an option,” he says. “We have more than 4 million people in the metro area ... where would they all possibly go even if we could get them out? [Houston is bigger than other cities in Texas, like Dallas, after all.] Putting hundreds of thousands of people on the road may have increased fatalities.”

Brian Wolshon, a civil engineer and evacuation expert at Louisiana State University, says such an evacuation is possible, but ideally “they occur while the skies are still blue.” There might not have been enough time to get people out of harm’s way. Though, Wolshon stresses, the question “was the evacuation decision right or wrong” never has a clear answer, and it’s easy to criticize in retrospect. “When you’re evacuating on that scale, it’s enormously complex, we can do our best to plan, but we can’t control people’s behavior.”

So was the no-evacuation call a good one? Bedient says the death toll, grimly, will give the final answer. So far, 11 people have been reported to have died in the storm. It’s a tragedy, and that number still may rise as the search effort continues. But it’s also a miracle so many survived, considering how thousands of people had to be rescued from their homes, and how this truly catastrophic storm descended on one of America’s most populated areas.

Though it’s not over. The rains will keep coming. On Tuesday morning, Texas Gov. Greg Abbott told CNN a thousand more may need to be rescued. The damage is likely to amount in the tens of billions. People have not yet been able to return to their homes. This story is going to continue for a long time.