One afternoon in late 2021, Microsoft’s chief executive, Satya Nadella, met with the commander of Israel’s military surveillance agency, Unit 8200. On the spy chief’s agenda: moving vast amounts of top secret intelligence material into the US company’s cloud.

Meeting at Microsoft’s headquarters near Seattle, a former chicken farm turned hi-tech campus, the spymaster, Yossi Sariel, won Nadella’s support for a plan that would grant Unit 8200 access to a customised and segregated area within Microsoft’s Azure cloud platform.

Armed with Azure’s near-limitless storage capacity, Unit 8200 began building a powerful new mass surveillance tool: a sweeping and intrusive system that collects and stores recordings of millions of mobile phone calls made each day by Palestinians in Gaza and the West Bank.

Revealed here for the first time in an investigation by the Guardian with the Israeli-Palestinian publication +972 Magazine and Hebrew-language outlet Local Call, the cloud-based system – which first became operational in 2022 – enables Unit 8200 to store a giant trove of calls daily for extended periods of time.