The reason they stopped here was because of the giant tree in front of them which had a natural tree hole. The current standards of a big tree are vastly different from the time before the apocalypse. It was very common to see big trees which had trunks that were one to two meters thick. Only those that were three to four meters thick were considered a big tree nowadays and the tree they found had a six-meter-thick trunk! That was the biggest tree they saw along their journey.
Luo Yuan had removed all the surrounding aerial roots and then told the rest to keep a distance. He walked to the tree hole and released a force which made numerous insects and small animals run out from the hole. That place looked like a mini ecosystem as there were many kinds of insects and small animals there.