Caliburn101
Explorer
fwiw: previous editions (on which 5e's version is based) said: "Half the sphere projects above the ground, and the lower hemisphere passes through the ground" even though "The interior of the hut is a hemisphere".
5e's unfortunate rewording created ambiguity, but I do not see an indication that the writers intended the spell to have a weakness to burrowing creatures (or creatures with access to items that could be used to shovel), and The Sage (Jeremy Crawford) has stated that the spell's effect do encompass the floor.
Allowing this vulnerability would seem to change the nature of this spell (in that, on earthen surfaces, most intelligent creatures could excavate a small entrance within minutes just by using their helmets, shields, etc.).
Past editions are moot as far as RAW is concerned, and in this particular case, Jeremy's opinion is irrelevant. The wording is very clear - with no mention of hemisphere, so underneath, it's vulnerable.