Anti-mafia police found the cargo in a shipping container on a Liberian-registered ship that docked in the Calabrian port of Gioia Tauro in southern Italy last month and said it would have been enough to destroy the port.
The cargo, hidden among packs of powdered milk, was seized on August 27 but police, who were tipped off by intelligence services, did not make an announcement until this week.
Carmelo Casabona, the local police chief, said the cargo arrived in Italy "with the involvement of international criminal organizations." But the 'Ndrangheta, the powerful Calabrian mafia, did not appear to be involved, he said.
Authorities were originally considering whether some of the explosives may have been intended to be unloaded in Italy for local use but they said this did not now appear to be the case.
RDX, sometimes known as T4, is a high explosive designed for military and industrial use.
