When spring comes, Mr. Robinson and Fritz go to check the supplies left at Tentholm. They find everything in tatters, the tent blown to the ground, their stored provisions wet, and their gunpowder ruined. The tent is clearly not a satisfactory solution. It is Fritz who has the idea to dig out a cave instead:

Fritz proposed that we should hollow out a cave in the rock.

Mr. Robinson and the boys work for days on end and get almost nowhere. Mr. Robinson is thinking they may have to settle for just carving out a storeroom for their supplies, when suddenly, Jack's crowbar disappears through the mountain. They discover that a thin wall separates them from a large a cavern and are able to break through. What they meet is a breathtaking and huge salt cave. Mr. Robinson states,

We were in a grotto of diamonds—a vast cave of glittering crystal; the candles reflected on the walls a golden light, bright as the stars of Heaven, while great crystal pillars rose from the floor like mighty trees, mingling their branches high above us and drooping in hundreds of stalactites, which sparkled and glittered with all the colours of the rainbow.

The place is solid and completely dry. They shoot off guns to make sure that pieces of the salt won't fall on them from the ceiling. They are delighted that this is a salt cavern, for they are equipped with all the salt they will ever need. They realize they can use this as their winter home.

They divide the large space into four areas so that they can live, eat, sleep, and store their supplies and animals. This provides a safe haven for them that Mr. Robinson is grateful for.