Cache Island is a small rocky island on the Lake. It is the northernmost of the islands off Rio (WH15). It appears several times in Winter Holiday. The explorers visit it on January 28 while waiting for Nancy to recover from the mumps, and leave a cache there with a message in a bottle noting it as the northernmost place they've gone (WH15). When the Ds are on their way to the pole on February 10, they stop at Cache Island and add a record of their progress to the message in the bottle (WH24). When Peggy and the Swallows are searching for the Ds that night, this message confirms to them that the Ds have passed that way heading for the pole; they leave a further message noting that the relief party has passed that way (WH27).