What is a rabbit thinking?
Thumping about something. You will not be told what.
The thump is an alarm, and it is meant for other rabbits. In a warren the sound carries through the ground better than through the air, so a rabbit that has seen something worth worrying about tells everyone underground at once, without making a noise a predator can locate.
Which is why you never find out what it was. The message was never addressed to you. Your rabbit has a threat model, you are not on the distribution list, and the thumping will continue.
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