Yeah, know where you're heading.
In Canada, four (4) feet minimum will clear a guy from the permafrost(lat=54N). The further north, the deeper you need to go. Eventually you'll hit bedrock, the Artic tundra has very little topsoil.
But tell you what, up North, little guys like me can hit a long ball just like John Daley. You get a lot of roll over those outcrops!
Kenneth J Hueston, PEng
Principal
Sturni-Hueston Engineering Inc
Edmonton, Alberta Canada