Less than 2 hours ago it was reported from people east of Tehran of a hearing and seeing a massive explosion. The Iranian government was slow to give an official explanation and when they did they said it was nothing more than a gas explosion. Typically Iran lies about such things to cover up any military hits against the nation. Power was out in the area, and there were additional rumors of other attacks on power stations south of Tehran.
Here are a few rumors from twitter accounts on what happened:
"It’s an ammunition dump. It’s an IRGC base. It’s a power plant. It’s the Israelis. It’s an accident. It’s in the west of Tehran. It’s in the northeast of Tehran.
What we know so far about the big explosion tonight in Tehran."
also:
People reporting smelling a metallic smell in the air.
Reports of a "terrible noise" prior to the explosion.
Iran has placed a news blackout about the blast.

There were rumors it was a nuclear facility that was bombed by Israel, or a missile plant was hit. Right now there are no verifiable reports on what actually happened.
Yesterday Sec. Pompeo announced that Iran was refusing inspection of a nuclear plant. It must also be noted that Trump had told the Israelis that if they ever attacked Iran that the US would back them up.

here is a twitter account that showed the explosion.
(scrool down)
https://twitter.com/wakaltyaqein