Walkers strolling on a beach near Hastings stumbled across some large packages – which turned out to be concealing a tonne of cocaine.
The packages, all in waterproof bags and attached to life jackets to make them float, had a combined weight of around 960kg, which if cut and sold on the streets in the UK would have had an estimated street value of £80m.
Two separate piles of bags were found, with some floating in the water off the coast of St Leonards in East Sussex.
Samples taken from the load have tested positive for cocaine, though a full forensic examination will now be carried out.
National Crime Agency officers have now opened an investigation.
Martin Grace, NCA Branch Commander, described the haul as "significant.
"We think it originated in South America, but we are keeping an open mind about how the drugs washed up here and where the end destination might have been," he said.
"Clearly though, losing a consignment of this size will represent a significant hit to the criminal networks involved.
"Our investigation is being assisted by both Sussex Police and Border Force, and is also likely to involve international partners."