Hurricane Harvey pummeled the Houston area with an unprecedented four feet of rain, making it the most extreme rain event in U.S. history. Cars and houses are underwater, and thousands of people have needed or await rescue.

The event has been called a "500-year flood" — a flood event that has a one-in-500 chance of occurring in any given year in that location. Residents in 500-year flood zones aren't required to purchase flood insurance, even though there is more than a 5 percent chance, more than one in 20, of a 500-year flood over a 30-year mortgage.

The Federal Emergency Management Agency (FEMA) mandates that people living in the high-risk 100-year flood zone purchase flood insurance. While a 1 percent chance every year sounds unlikely, consider that it equates to roughly a 25 percent chance over a 30-year mortgage. A 500-year flood has roughly a 5 percent chance.

Hundreds of thousands of people live in flood-prone cities like Houston, Tampa Bay, Miami, New Orleans and New York.

In catastrophic flood events like the one in Houston, areas well beyond the 100-year flood line, or even the 500-year line, can end up under water. (Recent reports claim up to 30 percent of Houston may be flooded due to Harvey.) For those without federal flood insurance, there are few resources to help them rebuild after the floodwaters recede.

Here's what 500-year floods look like, or could look like, in other cities.

Tampa Bay

Washington Post The Tampa Bay area is the most vulnerable in the United States to flooding and hurricane damage.

Washington Post

The Tampa Bay area is the most vulnerable in the United States to flooding and hurricane damage.

Analysts say the Tampa Bay area, which includes the cities of Tampa and St. Petersburg, is the most vulnerable in the United States to flooding and damage in the event of a major hurricane. A direct hit would likely surpass the cost of Hurricane Katrina, with one Boston firm that analyzes potential catastrophic damage estimating a cost of more than $175 billion to the region. More than 30 percent of residents live in a moderate to high risk flood zone.

Unlike the other areas included, the Tampa Bay region hasn't experienced a direct hit from a Category 3 or higher hurricane in nearly a century.

Miami-Dade County

Washington Post Nearly half of all people in Miami-Dade County live in a high-risk flood zone.

Washington Post

Nearly half of all people in Miami-Dade County live in a high-risk flood zone.

Nearly half of all people in Miami-Dade County live in a high-risk flood zone. While the last major hurricane to make landfall nearby was Hurricane Andrew in 1992*, sea-level rise has contributed to higher king tides, or the highest tides, and more frequent coastal flooding.

Some areas of Miami-Dade, like the "Billion Dollar Sandbar" in Biscayne Bay, lie on reclaimed land only a few feet above sea level. A large chunk of the county's western side is uninhabited wetlands outside the county's development zone.

Harris County, Texas (Houston)

Washington Post Harris County, Texas, Houston: Almost 13 percent of people live in a 100-year flood zone in Houston and Harris County, but as one of the nation's most populous counties, that's more than half a million people.

Washington Post

Harris County, Texas, Houston: Almost 13 percent of people live in a 100-year flood zone in Houston and Harris County, but as one of the nation's most populous counties, that's more than half a million people.

Almost 13 percent of people live in a 100-year flood zone in Houston and Harris County, but as one of the nation's most populous counties, that's more than half a million people. Another half-million residents live in moderate risk areas, the 500-year flood zone, where the likelihood of flooding is between 1 and 0.2 percent each year.

This works out to over a million people in moderate-to-high flood-risk areas in Houston alone. Other cities that have recently weathered hits from hurricanes aren't much better off.

New Orleans, La.

Washington Post With much of the city below sea level, New Orleans is particularly vulnerable to flooding.

Washington Post

With much of the city below sea level, New Orleans is particularly vulnerable to flooding.

With much of the city below sea level, New Orleans is particularly vulnerable to flooding. Hurricane Katrina devastated the city in 2005, and remains the most costly and one of the deadliest hurricanes to hit the United States. More than ten years later, the city's population still hasn't recovered to pre-Katrina levels.

Nearly 95,000 people live in areas at a high risk of flooding, requiring flood insurance, while another 160,000 live in moderate-risk areas. The rest of New Orleans's population resides in areas protected from floods by levees, though in the event of a levee failure, as in Katrina, those areas would also become vulnerable.

New York, N.Y.

Washington Post Only about 7 percent of New York's population lives in a flood area that is moderate- to high-risk. But that amounts to more than 560,000 people.

Washington Post

Only about 7 percent of New York's population lives in a flood area that is moderate- to high-risk. But that amounts to more than 560,000 people.

New York City sustained an estimated $4.8 billion in uninsured private losses as a result of Hurricane Sandy in 2012, which caused a storm surge that flooded the subway system and shut down the New York Stock Exchange. Only about 7 percent of the city's population lives in a flood area that is moderate- to high-risk. But because of New York's size, that amounts to more than 560,000 people - greater than the number of people living in flood zones in Tampa Bay. The places most vulnerable to flooding are along the city's waterfronts in all five boroughs.

* This article has been edited to reflect a correction. Hurricane Andrew hit South Florida in 1992. An incorrect year was used before.