There is 1ft of chicken wire on the ground all the way around the inside of the fence which makes is difficult to dig out. That, and the large amount of space plus the mound for burrows means they don't try to escape.
Discussion
Thanks! Is there any back of the napkin calculation regarding how much space is optimal?
This article should give you a good idea of size and design. We only keep 3 breeding does and one buck in our colony. Our colony is much larger than the example in the article, though I'm not exactly sure how large.
https://www.motherearthnews.com/homesteading-and-livestock/raising-rabbits-zmaz77jazbon/