I only know of earthworm that they have been found up to 7 meters deep. But the rabbit hole thing is definitely not about the depth but about Alice in wonderland and the strange things you might find down there.
That sounds like some pretty dedicated earthworms! Also, as noted in another comment about the Alice reference, that’s a good point, albeit I guess one could say it’s about more of a figurative depth than literal given the curious places they go!