waves generated by winds a long way away, possibly arriving at shore without local winds
2005: Year-round the powerful ground swells generated in the notorious Roaring Forties 1000 kilometres to the south of Cape Leeuwin march north-east and meet with the reefs and beaches of the region. — MargaretRiver.com.
a broad and deep undulation of the ocean an obvious change of public opinion or political sentiment that occurs without leadership or overt expression; "there was a ground swell of antiwar sentiment