The ocean covers more than 70 percent of our planet, an area of over 160 million square miles. It is so immense that explorers once thought there was no way to cross it. When our ships were advanced enough to do so, naturalists then thought it impossible for humans to ever exhaust fisheries or drive marine species to extinction.

They were wrong.

Commercial fishing now covers an area four times that of agriculture, and much of that expanse has been rendered completely unsustainable. We have depleted 90 percent of formerly important coastal species. Large fish have been harvested so heavily that they are virtually wiped out in many places. Indeed, studying once vital fish habitats such as coral reefs has been compared to trying to understand the Serengeti by studying termites and locusts while ignoring the wildebeest and lions.

Some may hope that there are immense areas still untouched, given that humans do not live on the ocean and we need specialized ships to go far beyond the coast. But that is incorrect. In a study published this summer in Current Biology, we used fine-scale global data on 15 human stressors to the ocean—including commercial shipping, sediment runoff and several types of fishing—to show that Earth's “marine wilderness” is dwindling. Just 13 percent of the ocean remains as wilderness, and in coastal regions, where human activities are most intense, there is almost no wilderness left at all. Of the roughly 21 million square miles of marine wilderness remaining, almost all is found in the Arctic and Antarctic or around remote Pacific island nations with low populations.

These remnants of wilderness are home to unparalleled marine life, sustaining large predators and high levels of genetic diversity. The lack of human impact can also make them highly resilient to rising sea temperatures and coral bleaching—stressors that cannot be halted without globally coordinated efforts to reduce emissions.

In an era of widespread marine biodiversity loss, wilderness areas also act like a window into the past, revealing what the ocean looked like before overfishing and pollution took their toll. This is crucial information for marine conservation. If we are to restore degraded areas to their former state, we need to know what to aim for.

What concerns us now is that most wilderness remains unprotected. This means it could be lost at any time, as advances in technology allow us to fish deeper and ship farther than ever before. Thanks to a warming climate, even places that were once safeguarded because of year-round ice cover are now open to fishing and shipping.

This lack of protection stems in large part from international environmental policies failing to recognize the unique values of wilderness, instead focusing on saving at-risk ecosystems and avoiding extinctions. This is akin to a government using its entire health budget on emergency cardiac surgery, without preemptive policies encouraging exercise to decrease the risk of heart attacks occurring in the first place.

If Earth's marine biodiversity is to be preserved in perpetuity, it is time for conservation to focus not only on the E.R. but also on preventive health measures.

Our Current Biology paper comes with a plea. As we develop international conservation agreements, it is crucial that we recognize the unique values of wilderness and set targets for its retention. Without further action, wilderness areas will likely be lost forever—something President Lyndon B. Johnson urged us to avoid when he signed the Wilderness Act in 1964. “If future generations are to remember us with gratitude rather than contempt,” Johnson observed, “we must leave them more than the miracles of technology. We must leave them a glimpse of the world as it was in the beginning.”