Undetected Infiltration is a World Quest that can be initiated in The Chasm. It involves helping Yanbo, a Millelith disguised as a Treasure Hoarder, infiltrate Treasure Hoarder camps around the area. This quest will reward you with a Quest Item for The Millenial Mountains, as well as an Achievement.
This quest can be triggered by defeating the group of Treasure Hoarders between Tiangong Gorge and Glaze Peak. Their exact location is circled in the image below.
After defeating the Treasure Hoarders, a cutscene will play. Yanbo will then ask you to meet him in Lumberpick Valley. Follow the quest marker to his location and speak with him. He'll ask you to clear our two Treasure Hoarder camps.
Follow the quest marker to both camps in Lumberpick Valley and defeat all the Treasure Hoarders in the area. You'll then be prompted to head to a larger camp, where you need to defeat multiple waves of Treasure Hoarders. Make sure to bring a high-DPS team, as you need to defeat tougher enemies this time around, including two "bosses": Brass Bull Zhao and Chen the Flower.
Completing all battles will prompt a dialogue with Yanbo, completing the World Quest. You'll be rewarded with the Cup of Commons and the Wonders of the World Achievement called Well Done, Stierlitz!
Check out our guide on How to complete the Millenial Mountains World Quest in Genshin Impact to make use of your Cup of Commons.