In the late evening of October 5, seismic tremors of a magnitude of 4.6 on the Richter Scale were detected in Iran's Semnan region. Although they could be felt even in the capital Tehran, over a hundred kilometers away from the epicenter, as earthquakes go this was not a major event: It was not terribly strong and caused no casualties. And yet it has attracted global attention. The reason is that we are not sure that it really was an earthquake.
Since the tremors shook the Iranian desert, speculation that this was, in reality, an underground nuclear test has not been dying down, in some traditional media and in social media everywhere. In Iran itself, according to the Tehran Times - an outward-facing English-language publication - "seismologists and [...] authorities" have denied a nuclear test. The newspaper added that "CIA Director William Burns also said there is no evidence that Iran has decided to build a nuclear weapon." Considering that, from long and bitter experience, Iranians do not generally consider the CIA a source of truth, that is an intriguing, maybe tongue-in-check addition.
It is not hard to imagine plausible reasons why the leadership in Tehran could have an interest in staging a test that it knows leaves detectable traces while at the same time it's officially denying that it has done so: it would, in essence, serve to warn enemies while allowing for a degree of politically flexible deniability. It would also, perhaps, create some strategic ambiguity - that is, uncertainty among opponents - if not about the event itself, then about what exactly the Iranian leadership is intending to do with it.
Comment: As can be seen with the example of North Korea, the fact that it is has nuclear weapons is likely the key it has not been attacked by the US and its vassals. If indeed Iran made a nuclear test on October the 5th, then it would help explain why Israel didn't attack immediately as they had announced.
Israel and the US want to take out Iranian nuclear program but have they pondered that Iran can retaliate and take out Israel's nuclear facilities apart from the US bases around the Middle East? That option is still there without Iran having to have nukes as Iran has missiles which can penetrate the Israeli defence shield.
In terms of size, Israel covers an area equal to 1,3% of Iran or in figures, ~21000 km2 versus 1,648,195 km2.