Within hours of Iran proudly announcing the launch of its latest centrifuges, a power blackout damaged some of the precious machines at its site in Natanz.
Iran has described this as an act of "terrorism" and pointed the finger at Israel. But there is still mystery over the cause.
In Israel, some reports have suggested a cyber-attack might have been responsible but Iran has talked of "infiltrators" amid reports of an explosion linked to the power generator.
One thing reports seem to agree on is that an "incident" affected the power distribution network at Natanz, leading to a blackout until emergency power systems kicked in.
Iran's nuclear crisis in 300 words
Iran Iran's nuclear crisis in 300 words‘to avenge Israeli attack’ on nuclear site
Why do the limits on uranium enrichment matter?
A blackout may not sound that serious, but it can be at an enrichment plant. Centrifuges are slender machines linked up in what are called cascades which enrich uranium gas by spinning it at incredibly high speeds using rotors. The stress on the advanced materials involved is intense and the process is technically immensely challenging.
A small problem can send a centrifuge spinning out of control, with parts smashing into each other and damaging a whole cascade.
Ensuring the power supply reaching a centrifuge is perfectly balanced is vital. Which means sabotage of that supply can be catastrophic.
The question is: what caused it - a cyber-attack or a physical act of sabotage, like a bomb?
The speculation of cyber-attack comes because Natanz is Ground Zero for cyber-war - the place where the world's first real cyber-attack took place a decade ago.
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