The ocean covers more than 70 percent of our planet, an area of nearly 140 million square miles. It is so immense that explorers once thought there was no way to cross it. Once 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.

How we have proved them wrong.

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

But surely, given that humans don’t live on the ocean and we need specialized ships to go far beyond the coast, there must still be immense areas that are untouched.

That is incorrect. In a study just published in Current Biology, we used fine scale global data on 19 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 marine wilderness areas are home to unparalleled levels of marine life, sustaining large predators and high levels of genetic diversity. Their lack of human impacts 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.

Despite being irreplaceable and increasingly threatened, most wilderness remains unprotected. This means it could be lost at any time, as improvements in technology allow us to fish deeper and ship further than ever before. Thanks to a warming climate, even places that were once safe due to year-round ice cover are now open to fishing and shipping.

This lack of protection is largely due to international environmental policies failing to recognize the unique values of wilderness, instead focusing on saving at-risk ecosystems and avoiding extinctions. This is like a government using its entire health budget on emergency heart surgery, without pre-emptive 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 emergency room but also on preventative health measures.

This paper comes with a plea. As we develop international conservation agreements, it is crucial that we recognize the unique values of wilderness and sets targets for its retention.

Without such targets, wilderness areas will likely be lost forever—something President Lyndon B. Johnson urged us to avoid when he signed the American Wilderness Act in 1964. “If future generations are to remember us with gratitude rather than contempt,” observed Johnson, “we must leave them something more than the miracles of technology. We must leave them a glimpse of the world as it was in the beginning.”