The average seashell found along a shoreline is fixed at the same frequency as the body of water that birthed it. When held to the ear, the same, steady undulation of waves can be heard, sliding in and out of time. For this reason, they make excellent souvenirs for tourists, providing instant access to memories of better climes.

Meanwhile, the spiraling apex of a black conch’s shell can be twisted in several places like a radio’s dial. The larger the mussel, the more precision that can be attained while tuning its abandoned hermitage. Hobbyists who collect them are willing to spend tens of thousands of dollars per specimen, as it is rare for more than two or three to be found in a single decade’s span. They are angular, obsidian things, and some are even as large as a buffalo’s skull.

A skilled operator can manipulate the shell’s various whorls to listen in on far more than its ocean of origin. Adjustments made to its crown can surveil a wide swath of space and time, drawing the signatures of elemental water inward along curves defined by its radial symmetry. Such explorers eventually master identification of the mundane seas and lakes of the present; the most experienced among their ranks have heard the waves that carried salt between Gondwanaland and Laurasia, as well as those rolling, silver mists that once wandered across the moon’s Sea of Tranquility. 

The professional scientist often dismisses these claims as absurd, noting that any seashell-sized chamber will amplify the ambient noise of its surroundings in a sort of auditory pareidolia. While this is a valid approach to the problem, it fails to address it in full, for in matters like these, noise is the signal that is sought.

No mollusk is stranger than the human heart.

Europa's oceans are often found to be too loud to detect.

Other waters are better left undetected.