Police have been removing slabs from the back garden of a house in Nottingham as part of an undisclosed investigation.
Nottinghamshire Police said the search at the property in Greencroft, Clifton, had been prompted by "intelligence the force has received".
They said the operation, which involved a handful of officers, was not linked to the current occupant of the house.
There is no risk to the public, the force added.
At the scene
Rachel Royce, BBC East Midlands Today
We know police started their dig about two days ago because I spoke to the person living in the house, who is not in any way connected to the investigation.
He said they had knocked on his door and told him that they were going to be doing this work but did not explain what it was for.
There's a skip full of concrete slabs which police have taken from the garden, clearly digging up the patio looking for evidence for whatever this is in connection with.
I've seen at least half a dozen officers going in and out, including some scene of crime officers, so it's clearly part of a serious investigation.
I've also spoken to some of the neighbours who have no idea what is going but are curious and not a little alarmed.