When you're diving, you crash through glass, shattered into a million pieces by the waves. The ocean's surface really is a glass border between two different worlds. From above the glass is dark, the waves are blue. From below, the glass is white, the waves shimmer with light. When you dive you pass, truly, into a bizarro world.
When I went under I looked up, at what was once down. It made no sense. I could see the sun, but everything around it had folded into an unintelligible circle. The ocean's surface really is glass in that sense, it forms a literal fish-eye lens, sucking in the sky from horizon to zenith.
From below, it looks a mess. The waves make an undulating hall of funhouse mirrors, each one moving with the wind. It's beautiful but impossible to comprehend. I looked for a minute and then that world just disappeared. Underwater, it really feels like above water doesn't exist.
I thought, as I turned and looked down. This is what fish must feel like. They must not give a fuck about us. Just as we don't give a fuck about them. Life on our side of the mirror is killing them. I can see it in the bleached coral from the ever warming seas. This isn't the first time death has come across the glass border. Two billion years ago, it was the oceans that changed the air.
It's called the Great Oxygenation Event or—depending on who you're asking—the Great Oxygen Holocaust. It's important to understand, because we're causing another holocaust today.