It is a special chest with random rewards. It is available to players with a Detective Pass who have already deciphered the diary. The Cache is on the big card to the right of the decoder. Keep collecting tokens to open it! Inside, you’ll find diamonds, energy, tools, and other useful items.
You can open the Bottomless Cache as many times as you like while the season is live, but note that each time you’ll need more and more tokens to do this.