Built in 1884 to provide water storage for Leyland, the reservoir was decommissioned 21 years ago.
Previously owned by United Utilities, it was sold at auction in March.
Historian Boyd Harris, who has written a commemorative booklet on the reservoir, described it as "like being in the hall of the mountain king".
English Heritage has twice refused an application for the reservoir to be made a listed building.
It said it does not fulfil the criteria for national recognition as it is "neither a rare nor an exceptional example of its type".
From Saturday, the reservoir will be open for two weeks for free guided tours between 10:00 BST and 16:00 BST.
When we got there the site was packed! So not an easy situation to take photos.... Good to see people giving a toss though.