A kettle is a geologic formation. It presents as a void or depression in the ground.
Characterized by its steep sides, it's the result of glacial action, possibly as late as the Pliocene-Quarternary era (from about 5.3 million years ago to the present day). Kettles are the result of a block of ice broke that broke from a glacier moving over the area, scouring the ground with its odd imprint, then melting after the glacier.
Trenchlesspedia Explains Kettle
A kettle may be a surface or subsurface feature. Because of the passage of time since a kettle's formation and subsequent geological changes to the surface, a kettle's presence is unlikely to be detected by simple potholing. This means a more thorough geotechnical investigation may be warranted, depending on the geological history of the area through which a trenchless design path is planned.
A subterranean kettle may present as a void or as a sudden change in the material through which a drill or auger cuts or bores. This may necessitate either a change in drill bit or drill speed to avoid damage op the bix or a fouled hole.