Pakistani customs officials say they have seized 1,600 kilograms of heroin - apparently making it the country's largest-ever haul.
Afghanistan's poppy fields produce most of the world's heroin
The drugs were discovered in a deserted mountain hide-out in Baluchistan province, near the Afghan border.
No arrests were made - the authorities say they were acting on a tip-off. Arms and ammunition were also found.
Pakistan is a key transit point for heroin from Afghanistan making its way to markets in Europe and beyond.
Experts say heroin production in Afghanistan has approached record levels since the ouster of the Taleban regime in 2001.
Poverty and warlords' might are blamed for the widespread cultivation of the opium poppy, which provides the raw ingredients for heroin.
Pakistani authorities say they have not calculated the street value of the heroin they seized on Thursday, but it was not of an especially high quality.