This cache is the bonus cache that can be found by placing the clues gathered in the first 4 stages of the Adventure Lab "The River by Purdy's Mill" into the formula given at the 5th stage of that Adventure Lab.
Please use stealth and be wary of muggles in the area of this cache.
N uvag fubhyqa'g or erdhverq, ohg vs lbh ner FGHPX whfg nfx.