If eels were an anadromous species like salmon, spawned in fresh water and spending their youth and adulthood in saltwater only to return when the mating frenzy was upon them, then this would be a relatively easy mystery to solve. These eels had found a source of freshwater and were trying to swim upstream to spawn. But that would be fake news!
The American eel is a catadromous species. It migrates from saltwater to fresh, only returning to its breeding grounds in the Sargasso Sea when it is adult and ready to mate. Rachel Carson described their incredible migration in her first book Under the Sea Wind.
So the mystery remains. Why would three adult American eels wiggle themselves up on the rocks on the night of a full moon in May? What watery scent drove them to their deaths? Is there some elixir in the pond water that drives eels mad with desire? Are they the succubi of the white lady who walks these shores? Only the Vaughan Cottage curator can answer that question.