This is Goliath, the krill. Don’t get too attached. Today this 1 centimeter crustacean will share the same fate as 40 million of his closest friends: a life sentence in the belly  of the largest blue whale in the world. Let’s call her Leviatha. Leviatha weighs  something like 150 metric tons, and she’s the largest animal in the world. But she’s not even close to being  the largest organism by weight, which is estimated to equal  about 40 Leviatha’s. So where is this behemoth? Here, in Utah. Sorry, that’s too close. Here. This is Pando, whose name means “I spread out.” Pando, a quaking aspen, has roughly 47,000 genetically identical clone trunks. Those all grow from one  enormous root system, which is why scientists consider Pando a single organism. Pando is the clear winner  of world’s largest organism by weight— an incredible 6 million kilograms. So how did Pando get to be so huge? Pando is not an unusual aspen  from a genetic standpoint. Rather, Pando’s size boils down  to three main factors: its age, its location, and aspens’  remarkable evolutionary adaptation of self-cloning. So first, Pando is incredibly expansive because it’s incredibly old. How old exactly? No one knows. Dendrochronologist estimates range from 80,000 to 1 million years. The problem is, there’s no simple way to gauge Pando’s age. Counting the rings of a single trunk will only account for up to 200 years or so, as Pando is in a constant cycle of growth, death, and renewal. On average, each individual tree  lives 130 years, before falling and being replaced by new ones. Second: location. During the last ice age,  which ended about 12,000 years ago, glaciers covered much  of the North American climate friendly to aspens. So if there were other comparably  sized clonal colonies, they may have perished then. Meanwhile, Pando’s corner of Utah remained glacier-free. The soil there is rich in nutrients  that Pando continuously replenishes; as it drops leaves and trunks, the nutrients return to nourish new generations of clones. Which brings us to the third cause of Pando’s size: cloning. Aspens are capable  of both sexual reproduction— which produces a new organism— and asexual reproduction—  which creates a clone. They tend to reproduce sexually  when conditions are unfavorable and the best strategy for survival  is to move elsewhere. Trees aren’t particularly mobile, but their seeds are. Like the rest of us, sexual reproduction is how Pando came into the world in the first place all those tens  or hundreds of thousands of years ago. The wind or a pollinator carried pollen from the flower of one of its parents to the other, where a sperm cell fertilized an egg. That flower produced fruit,  which split open, releasing hundreds of tiny, light seeds. The wind carried one to a wet spot of land in what is now Utah, where it took root and germinated into Pando’s first stem. A couple of years later, Pando grew mature enough to reproduce asexually. Asexual reproduction, or cloning, tends to happen when the environment  is favorable to growth. Aspens have long roots that burrow  through the soil. These can sprout shoots that grow up into new trunks. And while Pando grew and spread out, so did our ancestors. As Hunter-gatherers who made  cave paintings, survived an ice age, found their way to North America, built civilizations in Egypt and Mesopotamia, fought wars, domesticated animals, fought wars, formed nations, built machines, and invented the internet,  and always newer ways to fight wars. Pando has survived many millennia  of changing climates and encroaching ice. But it may not survive us. New stems are growing to maturity  much more slowly than they need to in order to replace the trunks that fall. Scientists have identified two main reasons for this. The first is that we’ve deprived  Pando of fire. When a fire clears a patch of forest, Aspen roots survive, and send shoots bursting up out  of the ground by the tens of thousands. And secondly, grazers like herds of cattle and mule deer— whose natural predators we’ve hunted to the point of local elimination— are eating Pando’s fresh growth. If we lose the world’s largest organism, we’ll lose a scientific treasure trove. Because Pando’s trunks  are genetically identical, they can serve as a controlled setting for studies on everything from the tree microbiome to the influence of climate on tree growth rates. The good news is,  we have a chance to save Pando, by reducing livestock grazing in the area and further protecting  the vulnerable young saplings. And the time to act is today. Because as with so many other marvels of our natural world, once they’re gone it will be a very, very long time before they return.