A massive operation is under way near the International border in Samba sector to unearth an underground tunnel which was suspected to have been used by four Jaish-e-Muhammad terrorists to sneak into the country from Pakistan, said officials on Sunday.All the four terrorists were killed in a gunfight when a Kashmir-bound truck, in which they were travelling, was stopped for checking at Ban toll plaza on Jammu-Srinagar national highway on Thursday.