He can't really hear Raylan from standing quite a ways behind him, but he thinks he heard Raylan saying one of them will be crossing it.

"Why are any of us going to the other side?" That wasn't the plan, was it? "No, we should try and get our new friend to come to our side." They don't know what's on the other side. Doc is pretty sure the fissure is not meant to be crossed, after what had happened to Five. It's far too dangerous for them to be attempting it.