For years, Israel and Iran have been involved in an escalating hidden war, a conflict played out in the shadows, on land, air and sea, with tit-for-tat attacks that avoided open clashes. But they are now becoming more conspicuous - with Iran's nuclear programme a major flashpoint. Iran's leaders say they have no plans to build nuclear weapons - but Israel claims otherwise. Numerous explosions in Iran's nuclear and missile facilities have been blamed by former Iranian officials on Israel's Foreign intelligence service, Mossad. The BBC's Jiyar Gol travels to Israel on the trail of half a ton of missing Iranian secret documents and investigates evidence suggesting they led to the assassination of an Iranian nuclear scientist.