Astronauts and cameras on the International Space Station frequently take stunning images of what cities around the world look like from space, but lights of a different color off the coast of one of these cities puzzled an astronaut recently. Fear not though, NASA has an explanation.

Reid Wiseman, an astronaut on the ISS, posted a photo of Bangkok, represented by the a blaze of white light at night. But what about those eerie green lights outside of the city in the ocean.

“No idea,” Wiseman wrote.

Image source: @astro_reid/Twitter

Image source: @astro_reid/Twitter

NASA picked up on some buzz about Wiseman’s photo and agreed with others that he was “likely seeing fishing boats, which use bright lamps to draw plankton, squid and fish to the surface.”

Universe Today also pointed to another NASA post published on the topic in 2013, which explained how squid fishermen in South America and southeastern Asia can use more than 100 lamps that generate up to 300 kilowatts of light on each boat.

“The satellite images are a tool to understand what is happening with the fishery, especially in international waters,” Ezequiel Cozzolino with Argentina’s national fisheries research institute told NASA at the time. “The images allow us to estimate the number of foreign jigging fleets that are fishing Illex argentinus, and to calculate the weekly captures of the species.”

Fishermen lights were first noticed from space in the late 1970s.

(H/T: Sploid)

Front page image via @astro_reid/Twitter.