Rabbits digging a burrow under shed

My rabbits have dug u burrow from one side of my 10ft shed to the other in literally 2 days now i think they've branched of and going god knows where,at first i thought its just rabbits being rabbits and its cool under there for them but yesterday bonnie was under there for 2 hours and when she did come out was black,also there was a lot of flys coming out of the hole,so im thinking ive got to stop them going under there or im going to loose a rabbit.

I would definitely stop them from getting in there because of the problem of not being able to get them out. If you had easy access to extract a bunny at will then I would say let them get on with it. But having had a bunny do this in the past, I'd stop it in the early stages now. Give them an area where they can dig without being able to get up to any mischief by all means but it's important that you can get to them at all times - especially if one becomes ill and decides to disappear into the burrow .

Yes, the trouble is they won't stop!

Having a safe area and blocking off the shed is the ideal option. You can make an accessible burrow by digging a trench and lining it with mesh or slabs or bricks, then making a roof from something you can lift eg wooden boards or slabs. That way then can chill underground and you can grab them in an emergency.