Superman’s Fortress of Solitude is the centre of attention again, in Action 411 (April 1972).
Superman discovers that the oil explorers have found the door to his Fortress, although they have no idea what it is. They are working to open it.
The head of the company insists that his lease on the area gives him the rights to whatever is found, and he seals off access to the Fortress, while trying a variety of methods to penetrate it.
Both Superman and Supergirl, who has her own wing there, are worried about their secrets being exposed, and so they sabotage the oil exploration rigs and boats, until staying becomes such a financial loss that the company just leaves.
Superman does offer to open the door for them before they go, and reveals a plaque on an ice wall. Enough to satisfy the people, but nor enough to maintain interest. Superman constructed this fake door and wall directly in front of the real one during the night.
From now on, the door and key are both concealed behind a hologram of ice.
A really acceptable update, with the increased activity in the far north by the time this story was written.