Hydrogen sulphide is the smell that comes from a rotten egg. Imagine that seeping out from a mound of rubbish the size of a small hill.
You don't have to imagine it if you go to the Staffordshire town of Silverdale, population 5,700. It's an ever-present threat and a frequent reality.
On this week's ClimateCast, Tom Heap visits Silverdale to speak to residents campaigning to shut down a landfill that's been haunting them for years.
He talks to protesters in the village, as well as the local doctors' surgery to ask health professionals if they believe the landfill is a risk to health - which operators deny.
Producers: Emma Rae Woodhouse and Mickey Carroll
Editors: Luke Denne and Philly Beaumont