Bomb disposal officers attended the scene, and police say a number of suspicious devices have been found. Streets were closed and a cordon put in place.
Around 40 houses on Edwina Street and nearby streets were evacuated. Residents were not expected back in their homes until this weekend.
That's everything we know.
Why this mattersSo was this a response to a threat? No.
Police have called it a proactive operation. That means it came from intelligence gathered beforehand, not a reaction to something that already happened.
Nobody has said an attack was about to happen. Nobody has said the public was in danger before police moved in.
But here's the thing. What they found once they got inside was serious enough to call in bomb disposal.
Officers who deal with bombs don't turn up for routine searches. And evacuating 40 homes isn't a precaution taken for its own sake. So whatever was in that house, police are treating it as a real explosives risk.
Now, the location matters too.
The Shankill Road carries a long and difficult history from the Troubles. A terrorism investigation centred on one house there will carry weight locally that goes well beyond what police have actually released.
And right now, that is very little. An arrest. A search. An evacuation. Nothing more.
Local politicians have reacted to the disruption, not to the investigation, because no detail of the investigation has been given.
Brian Kingston, the DUP MLA for North Belfast, says quote "deeply troubling".
That's a local politician saying this has rattled the area, without knowing any more than residents do.
Nicola Verner, a DUP councillor, says everyone is safe, with a large cordon in place and bomb disposal experts dealing with the incident.
Ian McLaughlin, another DUP councillor, calls it unwanted disruption, and says the community is rallying around those affected.
Police have not named the man. They have not said what the specific allegation is. They have not said whether it's linked to any group.
Under the Terrorism Act, a suspect can be held for an extended period before any charge comes. So this could run on for some time before anyone knows more.
He has not been charged. Nothing has been proven against him.
What happens next depends on two things. What forensic teams find inside that house. And whether the man in custody is charged, released, or held further.
Police say the investigation is continuing.
Right now, dozens of families from one Belfast street are just waiting to find out when they can go home.
