That cellar in linked to a quest in the DLC Broken Steel. This quest in given to you have the main questline in completed (the DLC lets you keep playing) This quest is named Holy Water (YOU WILL ONLY GET THIS QUEST IF YOU DID NOT DESTROY MEGATON) But to answer you question the key is found on a man named Brother Gerard, If you didnt destroy megaton, asked scribe bigsley at the Water Purifier for the quest, asked around megaton about the cult, then you should see this guy hanging around the cellar, he has the key.
It is on the top floor of Springvale Elementary, and requires you to pick a lock or use the Springvale Library Key (dropped by one of the raiders in the school). Springvale Elementary itself is near Vault 101 and Megaton.
The address of the Springvale Public Library is: 443 Main Street, Springvale, 04083 1819
The address of the Sanford Springvale Historical is: Po Box 276, Springvale, ME 04083-0276
The phone number of the Sanford Springvale Historical is: 207-490-1028.
There isn't a key for it unfortunately.
The area of City of Springvale is 97.6 square kilometers.
Springvale White Eagles was created in 1975.
Springvale railway station was created in 1880.
34 miles
To free the prisoners fromthe cellar of torment you must first defeat Jailor Matrok. He will give you a key that will unlock the cellar to Suri Mah. After that follow the blue arrows to the exit and then you have completed the mission.
in springvale ( chingvale)
he is the awesomest personn ever