The Glowing Sea

The Glowing Sea is a main story quest in Fallout 4.

Quick walkthrough
Find Virgil in the Glowing Sea.

Detailed walkthrough
Obtain a hazmat suit, walk southwest into the Glowing Sea to the Crater of Atom (denoted by the dark spot on the map). Talking to the leader of the Children of Atom will point you further southwest. After a conversation with Virgil he will tell what you need to know, and start Hunter/Hunted.