On casting ‘Imprisonment / Chaining’, on changing &/or non-stable surfaces – where, where do the chains go?

The 9th level spell Imprisonment can be cast a few different ways. One possible cast-version creates ‘chains’ that keep the target restrained:

You create a magical restraint to hold a creature that you can see within range. The target must succeed on a Wisdom saving throw or be bound by the spell; if it succeeds, it is immune to this spell if you cast it again. While affected by this spell, the creature doesn’t need to breathe, eat, or drink, and it doesn’t age. Divination spells can’t locate or perceive the target.

Chaining. Heavy chains, firmly rooted in the ground, hold the target in place. The target is restrained until the spell ends, and it can’t move or be moved by any means until then. The special component for this version of the spell is a fine chain of precious metal.

Note that these special Imprisonment-Chains are firmly rooted in the ground (R.A.W. / above). Here are my variants on one concern, below:

  • what if the target is 60′ to 120′ up… on a wooden floor? Do the chains reach all the way to ground level as the spell takes effect?

  • what if the imprisoned is standing on any firm yet non-stable surface when this spell is cast? Examples abound: on a ship, in the air due to a kite or hot air balloon. Or even on an otherwise quasi-stable surface such as sand or ice or swamp or lava or glass – the chains would have to take root somewhere ‘in the ground’.

I admit that this question isn’t deeply serious. Still, i am darn curious where these crazy chains would end up going.