Council chiefs say they have "serious concerns" over the site, an entire railway cutting in Haghill, Glasgow, which has been filled in with rubbish over recent months.
Local residents say up to 40 trucks a day are tipping waste into the mile-long 30ft-deep cutting, which is now almost level with the surrounding land.
A private landowner is understood to have bought the site and has permission from the Scottish Environment Protection Agency (Sepa) to "store" waste there.
However, the council insists there is no planning permission in place for the huge earth-moving operations on the site.
Local councillor Frank Docherty said: "The landowner needs to put a stop to this dumping immediately while our planners and Sepa take a closer look at what has been happening.
"Communities can pay a high price for this kind of behaviour."
The cutting appears to have been filled with rubble and earth from building sites. However, residents said tyres, fridges and TVs had been dumped on the site, although it is not clear by whom.
Glasgow City Council's planning enforcement team is leading the investigation.
A council spokesman said: "This is an issue we are aware of and, in terms of planning consent, it is a matter of serious concern."
People living close to the cutting said they had been given no warning it was to become a dumping ground.
Susan McGurk, 52, of Appin Road, used to have the deep cutting at the end of her garden. Now it has been infilled right up to her fence.
"We didn't get told anything," she said. "One day diggers and trucks appeared. Now we can get 40 a day, sometimes 10 queuing up with builders' rubble."
A Sepa spokesman said officers are investigating following complaints about the waste at the site.
He said: "The site is regulated by us under a registered exemption for the storage of specific waste materials used for construction purposes."
Action would be taken if exemption conditions have been breached, he added.