[mass noun] Subsoil.
- ‘The undersoil heating has been on all night and even though there was a covering of snow this morning, it should be okay.’
- ‘Which was the first British club to install undersoil heating?’
- ‘I just wish they'd put in undersoil heating when the ground was built.’
- ‘The addition of undersoil heating would almost rule out the postponement of matches for climatic reasons.’
- ‘The invention is modelled on the undersoil heating systems at football grounds.’