White foam on the Abuja‑Kaduna Expressway has drawn crowds and sparked an investigation by Nigeria's environmental agency.
Hundreds of motorists stopped to photograph the strange formation that appeared a few days ago.
The National Environmental Standards and Regulations Enforcement Agency (Nesrea) warned the public to keep a safe distance while it collected samples of the foam, nearby soil and stream water for laboratory testing.
Preliminary findings point to a chemical spill following a lorry accident, though the lorry's cargo remains unclear.
Officials urge people to avoid the substance until the investigation is complete.
