There is perhaps no movie with better-executed twists than 2019’s Parasite, the South Korean thriller directed by Bong Joon-ho.

But the movie’s greatest trick didn’t rely on sleight-of-hand camera techniques or elaborate set design. Instead, it takes a page out of a real-life climate disaster facing Seoul today. (Spoilers ahead for Parasite).

In one of the movie’s climactic scenes (pun only slightly intended), excessive rainfall causes flash flooding in the Seoul district of Ahyeon-dong, where the poor Kim family leaves. An errant open window causes the floodwaters to pour into their semi-basement dwelling, ruining their apartment. Desperate residents of the neighborhood use buckets to try to save their homes.

It’s a gut-wrenching moment, but also eerily prescient: this summer, record-breaking levels of rainfall pounded Seoul, flooding these semi-basement dwellings and causing the deaths of at least 14 people.

But just what are these semi-basements, and how does their vulnerability to flooding expose divides between the rich and the poor in one of Asia’s most expensive cities — in both real life and Parasite? Let’s get into the reel science behind this wild thriller.

Reel Science is an Inverse series that reveals the real (and fake) science behind your favorite movies and TV.

How does Parasite mirror real-life?

We’re first introduced to the poor Kim family in their humble semi-basement dwelling, known in South Korea as “banjiha,” which literally means “half-buried.” The phrase refers to the fact that these dwellings exist mostly below street level, but they can almost feel like they’re above-ground.

These semi-basements were originally constructed in the 1970s to serve as bunkers in the event of military clashes with North Korea, but later became repurposed as low-income residential housing. In a city where the average one-bedroom home now costs close to $900,000, home ownership is out of reach for Seoul’s poorest residents, many of whom can only afford to live in the banjiha.

Rents in these semi-basements — about five percent of the city’s housing stock — typically go for around $450 per month, according to the BBC. Roughly 200,000 semi-basements exist in downtown Seoul, including in the lower-income neighborhood of Ahyeon-dong, where the Kims live in the movie. But people living in banjiha pay a price for that lower cost, including societal stigma and physical and mental health issues due to the lack of sunlight and the mold that often grows due to humid conditions.

And, of course, there’s the flooding. In 2010, Seoul experienced serious flooding, which damaged more than 9,000 semi-basements. The city vowed to prohibit the construction of new banjiha, but developers continued to build them regardless.

Then, one night earlier this summer, record-breaking rainfall hit Seoul, causing nearly four inches of rain to fall each hour, even exceeding five inches at one point — the worst rainfall the city had experienced in 80 years. The city’s Han River was totally overwhelmed, and many semi-basement dwellings flooded in a scene that could have been straight out of the movie. Tragically, a family living in the banjiha died after they became trapped in their homes; rescuers couldn’t get to them until they had drained the apartment, at which point it was too late.

Why is Seoul vulnerable to flooding?

South Korea experienced an economic boom — and significant population growth — over the last several decades, leading to the development of dense urban housing in the city of Seoul, which is home to nearly 10 million people. Natural land was razed to build this dense housing, which was a problem because green infrastructure like vegetation and green spaces can help soak up rainfall — unlike asphalt roads — and reduce flooding.

According to a 2018 paper in Urban Design and Planning, developers also concentrated newer housing in lower-lying areas more prone to flooding. The authors argue that “the new areas were also developed without adequate consideration for stormwater facilities, resulting in repeated flooding.”

In Parasite, the wealthy Parks are deluged with rainfall, but their higher-elevation home makes the rain a mere annoyance rather than the life-changing nightmare it is for the Kims.

Just like in the movie, Seoul’s residents don’t experience the risk of flooding equally. While wealthier residents in expensive areas of Seoul like Gangnam did experience flooding in the recent storms, they didn’t face the same level of serious risk as lower-income individuals living in below-ground apartments.

A 2022 preprint study discusses how floods expose the deep levels of environmental injustice in South Korea, which has one of the highest rates of income inequality of the 38 member nations in the Organization for Economic Cooperation and Development. The paper states that those most at risk of flooding in Seoul tend to be vulnerable populations like people on public assistance, elderly individuals, and people living in semi-basement dwellings.

The future of urban flooding and climate change

Following the recent floods, the South Korean government once again declared a ban on new banjiha and stressed the need to provide more affordable housing. But they also mentioned something they didn’t discuss as much in the 2010 floods: climate change.

“It clearly is an unusual weather situation. But we cannot just say this kind of unusual climate situation is unusual. This kind of unusual situation can and will be repeated at any time,” South Korean President Yoon Suk-yeol said according to an NBC News report.

But Seoul is hardly the only city to face significant flooding in recent years. Last year, the remnants of Hurricane Ida caused sudden floods throughout New Jersey and New York, leading to the deaths of several residents who lived in basement units — not unlike the banjiha in Seoul.

Climate change makes extreme rainfall over a short period more likely as storms become wetter. Essentially, warmer temperatures in the ocean trap moisture in the air, so by the time it finally rains, it pours — hard. In a previous interview with Inverse, Alison Branco, New York Coastal Director and Acting Director of Climate Adaptation for the nonprofit The Nature Conservancy, spoke about New York’s likelihood of increased precipitation, which places like Seoul also face:

“We’re not expecting a lot more rain in terms of annual volume, but what we are expecting is for the rain to come in much more, kind of big, dramatic storms,” Branco said.

Korean scientists have looked into ways to make the urban landscape more resilient to flooding, like adding green spaces that absorb water runoff. Researchers in a 2016 study said that green spaces could help reduce flood risk by over 50 percent in Seoul depending on the location.

It’s also likely that the harrowing summer floods helped city official prepare residents for the severe rainfall of September’s Typhoon Hinnamnor — the strongest typhoon in Seoul’s recent history — and reduce casualties.

But experts also stress that there’s only so much we can do to adapt to pronounced flooding as a result of the climate crisis. Recent research reveals a staggering 1.8 billion people worldwide face once-in-a-century flood risks. Sea levels are going to rise even higher without further action, and supercharged typhoons will continue to hit Seoul in the future.

“There is nothing we can build that will stop flooding caused by sea-level rise,” Branco said.

Parasite’s flooding isn’t simply a movie fabrication, but, instead, an ominous warning of South Korea’s climate future — and the rest of the world.

Parasite is streaming now on Hulu.