Underground actually just does not emit much magnetic field. The cables are so close to eachother, that they counter their fields. "The magnetic field is weaker directly under an overhead line than directly over an underground cable. On the other hand, the reach of the magnetic field is lower from an underground cable because the arrangement of the cables partially cancels out the fields." (https://www.swissgrid.ch/dam/jcr:417fefcd-f488-4a62-ab48-e489b6d5d805/overhead-line-underground-cabling-en.pdf)