Snorkelling while the time is right. If you want to snorkell on some decent coral reefs do it now because in 20 years there won't be any left according to Roger Bradbury, a resource management ecologist at the Australian National University. Within a human generation they will collapse, he says. There will be remnants here and there but the global coral reef ecosystem will cease to be.

Over fishing, ocean acidification and pollution are pushing coral reefs into oblivion. Each of these forces alone is capable of causing the global collapse of coral reefs; together they assure it. Each is growing broadly in line with world economic growth.

Global fishing pressure is still accelerating; it is set to double in a decade even as the global fish catch is declining each year.

Ocean acidification is yearly more extreme because of increased carbon dioxide absorption from the atmosphere. Coral can make their calcareous skeletons only within a special range of temperature and acidity. That range will be exceeded in the next 20 to 30 years.

As for pollution, coral reefs can't survive in nutrient-rich waters. The only things that are encouraged are microbes soaking up the sun's energy by photosynthesis and lots of jellyfish feeding on them. What will be left is a slimy algal-dominated hard ocean bottom.

So remember: the underwater photographs that you take today could be of historic interest tomorrow.