But El Patrón is also remembered by more than 50 hippopotamuses (Hippopotamus amphibius) that currently roam free near his palatial estate, Hacienda Nápoles. Escobar's captive hippos were never meant for the rivers and estuaries of northern Colombia, yet since his death they've behaved as wild animals are wont to: by vigorously breeding and multiplying, slowly establishing themselves as the largest invasive species in the world.

When drug kingpin Pablo Escobar was killed by the Colombian National Police in 1993, he left a vast and bloody legacy in his wake. The Medellín Cartel boss is regarded as one of the most prolific criminals in history, and is notorious for having built a cocaine-fueled empire on the bodies of thousands of murdered individuals.

Today, it appears their troublesome reign is nowhere near ending because no one really knows how to stop them.

"We're observing a scary natural experiment of what the world's largest invasive animal can do to its new environment."

"This is all speculative business right now. We have a lot of historic ecological analogs for things that originally came from Africa and were eventually found in the New World—like the extinct American lion, or relatives of elephants—but hippos are just not in that portfolio," Douglas McCauley, a biology professor and hippo researcher at the University of California, Santa Barbara, told me.

But the private administrators who currently operate Hacienda Nápoles as a theme park aren't eager to cull or move the pachyderms. In 2009, a bull named "Pepe" was shot and killed by Colombian Army soldiers, igniting debates between rightfully concerned ecologists and those who view the charismatic megafauna as harmless curiosities. Scientists who study South America's rivers are worried the hippos might one day topple the region's delicate ecosystems.

Over the 30 years following Escobar's death, herds have reportedly broken through the compound's fences into nearby waterways, such as the Magdalena River. Last year, one of them was spotted meandering around a local elementary school. And although government officials assure that no human casualties have occurred, farmers and fishermen working in Puerto Triunfo are afraid to go near them.

However, Escobar also viewed himself as a benevolent dictator, and was notably celebrated by the poor residents of Medellín for sharing narco money to build communities and housing projects. So instead of merely living at Hacienda Nápoles, the man constructed a public zoo —a bootlegged menagerie filled with smuggled exotica such as elephants, giraffes, and zebras. Among the animals that schoolchildren and spectators traveled to see were four large hippos, three females and one male, that inhabited a small artificial lake near the entrance of the property.

Tucked away in the lowlands of Antioquia, some three hours outside of Medellín, Hacienda Nápoles persists as a bizarre shrine to Escobar's wealth and excess. At the height of his success, the drug lord's estimated net worth was approximately US $30 billion, which he spent on a predictably extravagant life. In 1978, Escobar purchased a parcel of land in the quiet hillsides of Puerto Triunfo and built a sprawling Spanish colonial mansion that would become his family's permanent home. The compound once featured amenities including a bullring, cart racing track, sculpture garden, and even its own private airport.

One survey estimated the hippos at Hacienda Nápoles will continue to grow at an annual rate of six percent, and every fertile female is expected to give birth to a new calf each year. Individuals have now been spotted more than 90 miles away from the drug lord's old compound.

For scientists like McCauley, Escobar's renegade hippos offer a bittersweet opportunity to witness what happens when the species is allowed to proliferate uninhibited. More than a millennium ago, diverse varieties of hippos, many of which are now extinct, were found as far north as the Mediterranean isles , Egypt , and even parts of Europe . Today, the common hippopotamus is found only in Africa, and is vulnerable to unregulated hunting, habitat destruction, and ivory poaching. Biologists who study the animal and its environment are seeing rapid population declines, and must now race to understand what the implications of a hippo-less ecosystem might be.

After Escobar was killed, most of the animals at Hacienda Nápoles were captured and transported to nearby facilities like the now-defunct Matecaña City Zoo in Pereira. The little hippo harem, on the other hand, was left behind, probably due to the fact that wrangling a 9,000 pound bull can be a life or death situation. So with no natural predators and a hospitable climate in which to thrive, four became eight, then 10, 20, and so on.

On the flip side, too much of a good thing (their poop) can be toxic. In cramped river systems, hippos can actually flood the water with excess nutrients, stimulating noxious algae blooms and starving fish and invertebrates of oxygen. This process, called eutrophication , has been linked to hippos before, and ecologists familiar with Colombia's waterways suspect that recent fish die-offs near Hacienda Nápoles could be a byproduct of the invasive residents.

In their native habitat, hippos can be described as nature's gardeners. Most of their day is spent submerged under water, shielded from the sun's harmful UV rays and possibly even humans. At night, however, hippos will emerge from their muddy squalor to feed on copious amounts of grass, and profusely poop out the previous evening's meal. Millions of pounds of hippo feces are deposited into Africa's rivers every year, and without this massive payload, they would be starved of life-sustaining nutrients. Scientists have used chemical markers to determine that many fish and insects feed off hippo dung—a practice known as "coprophagia"—which suggests the species plays a key role in connecting aquatic and terrestrial ecosystems.

"Here's the thing with invasives: we've seen all types destroy ecosystems and totally rearrange their ecology, changing everything about how they work and look. There's no reason to think that just because hippos are charismatic they should get any exceptional treatment as far as how we try to manage them," said McCauley.

As of right now, very little scientific literature on the effects of invasive hippos exists, simply because there's zero precedent for it. Shurin added that researchers often face a host of impediments to working in Colombia, and the issue of Escobar's escapees appears to have received minimal attention within the scientific community. Furthermore, the country is already grappling with broader ecological crises, such as illegal dumping , deforestation, and contaminated water . How dire of a threat are several dozen hippos?

"Hippos can also really disturb the sediment in lakes and rivers, which causes stored sediment to come back into the water column. This has a big effect on productivity, and can result in all kinds of consequences. Whether or not their impact on ecosystems is similar or different in Africa—everything they coexist with has evolved in their presence," Jonathan Shurin, a biology professor at the University of California, San Diego who studies water quality issues in Colombia, told me.

But as is the case with some invasive species, people became attached to them. Many local residents have allegedly grown fond of the amphibious pachyderms, and are at the very least captivated by them. "My father brought a little one home once," a young girl reportedly once told the El Colombiano newspaper . "I called him Luna because he was very sweet—we fed him with just milk."

Yet, herein lies the problem: There's no really effective way to control a rogue hippo population. According to Michael Knight, a zoologist at the South African Parks Unit who consulted for Colombia's Environment Ministry in 2009, only culling or castration are realistic options. In an interview with Colombia Reports, Knight recommended the animals be shot with high-powered rifles.

It's unclear how involved the Colombian government currently is with population management efforts, but at some point, castration and containment were adopted as the sole mechanisms for curbing the region's hippo numbers. Local environmental authority Cornare recently dispatched biologists to build natural barriers for the animals, aimed at confining them within the Hacienda Nápoles estate. The initiative operates on an annual budget of $135,000, and is entirely funded by money seized in drug raids.

In 2014, according to a report from Fusion, staff at Hacienda Nápoles were also asking a local man named James Torres to capture and foster baby hippos at his nearby farm. Torres remarked that separating the calves from their herds would make it easier for them to be transferred to zoos.

Castration, while ostensibly a happy medium, is proving too difficult for even the most seasoned of wildlife managers. For starters, hippos are surprisingly cryptic for their size. During the day, only their ears, eyes, and nostrils can be seen above water. Furthermore, hippos aren't sexually dimorphic, which means it's difficult to distinguish males from females. A male's testicles are hidden within its abdomen, and can only be safely identified once the animal is sedated. But darting a several-ton creature is a tricky process, McCauley told me. Even though the sedative cocktails used by veterinarians have become fairly sophisticated, it's impossible to know how each individual will react to anesthesia. Occasionally, a drugged hippo will become so scared it charges back into the water at the risk of drowning.

So far, only four bulls have been successfully castrated.

"With most invasives, once the cat's out of the bag there's no going back, but with hippos that's probably not the case. If they really wanted to remove them, they could. It seems the hippos have become a tourist attraction and have some economic value. The will is not there to remove them," added Shurin.