CONCORD, Mass. – The sudden appearance of freshwater jellyfish at historic Walden Pond has researchers at the New England Aquarium puzzled.

Scientists say the species, that probably came to the United States from Asia about a century ago, are spotted sporadically in ponds and lakes, but rarely in the numbers seen recently at Walden.

Researchers first got word of the dime-sized jellyfish from a woman who swims at Walden regularly. She had never seen them in 15 years of swimming.

The jellyfish pose little or no threat to humans. Aquarium curator of fishes Steve Bailey told The Boston Globe the jellies are “wickedly cool” critters.

The pond is famous for Henry David Thoreau’s 1854 work “Walden,” a social critique and commentary on nature that he wrote after living in solitude near the pond.