SAN DIEGO — The ocean and the sky melted together in a gray mass, making it tough for the surfer to size up the coming waves. Would they crash down on top of him and hold him under — or break in just the right spot for him to catch the perfect ride?

By day, the surfer, Helmut Igel, is not fazed by six-foot waves. But it was after midnight, on a moonless sea. Surfing is not the same in darkness.

Igel, 55, is among a small subculture of surfers who dot coastlines from San Diego to Sydney after sunset, pursuing an adventure that’s a subject of curiosity on social media these days thanks to pro surfers on LED boards. Visibility is but one of the perils.