Athos

"There is still talk of returning, even though I've told them the Wraith have scorched Athos bare in revenge for our resistance."

- Teyla Emmagan

Athos was the first planet the Atlantis Expedition visited after arriving in the Pegasus Galaxy. They met the Athosians, who told them of the Wraith.

The Athosian village was near a city built by the Ancestors. Teyla Emmagan took Major John Sheppard to some caves where the early Athosians hid from the Wraith, and where Sheppard found a necklace that belonged to Teyla. Unfortunately, the central locket was actually a Wraith transmitter that detected the ATA gene in Sheppard, which lead the Wraith to cull the planet. The Tau'ri helped the Athosians to fight back, but there were still a number of losses, leading Sheppard to take the Athosians back to Atlantis. As punishment for the Athosians defiance, the Wraith devastated the planet.

It was later revealed that at some point the Expedition must have returned as Teyla commented that the Wraith had "scorched it bare" in revenge.

Behind the scenes
The planet is most likely named after Mount Athos, a peninsular in Northern Greece.