Scuba divers underestimate how much damage they do to coral reefs. We've watched hundreds to find out why Lisa Lock Scientific Editor Andrew Zinin Chief Editor Scuba diving is often presented as a success story for marine ecotourism. Each year, millions of people dive beneath the surface to experience coral reefs firsthand.

Scuba diving generates income for coastal communities and helps people understand why coral reefs matter. But that success also brings a conservation challenge. It brings tourists into direct contact with sensitive coral ecosystems, where a misplaced fin, hand or camera can add to the stresses reefs already face.

At a time when reefs are severely threatened, understanding how divers might help or harm those ecosystems has never been more important. Between us, we have spent thousands of hours underwater as both dive professionals and marine scientists. In this time, we realized something uncomfortable: Even people who admire and value coral reefs can unintentionally contribute to their decline through their actions underwater.

That observation led us to ask a simple question: How much impact are divers actually having on coral reefs underwater—and do they realize it? For our research, we followed hundreds of recreational scuba divers, carefully documenting their behavior underwater. We then surveyed those same divers afterward to compare what they thought they had done with what had actually happened.

We spent hundreds of hours following, observing and surveying 732 scuba divers across world-famous dive destinations in Indonesia and the Philippines. We recorded every time a diver touched the reef and what was happening at that moment. The results surprised us.

Divers touched or hit the reef, on average, about once every four minutes. More than 40% of those contacts caused visible damage, including broken corals or sediment deposited onto living coral. Perhaps most strikingly, we found that most damaging contacts were accidental and often went entirely unnoticed.

Divers in our study also expressed strong environmental values, and many saw themselves as better than average at controlling buoyancy and avoiding reef contact. At least 69% of divers responding to our survey rated themselves "above average" in their ability to avoid intentional reef contacts, and 86% as "above average" in their ability to avoid accidental contacts. But when we compared their survey responses with our underwater observations, divers underestimated how often they contacted the reef by almost fivefold.

The divers making the most contacts also tended to be the most overconfident about their underwater performance. This pattern is consistent with the Dunning–Kruger effect, a well-established psychological phenomenon in which people with lower skill levels are often least able to recognize their own limitations. For coral reefs, this creates a conservation problem: People may not change their behavior if they do not realize they are part of the problem.

Influenced by others Underwater behavior was also surprisingly social. When one diver hit or touched the reef, nearby divers became more likely to do the same.