Quad bikes and scramblers are 'scarring the Mournes'
The throaty roar of the engines was the first thing that alerted us to the presence of the bikes; then we saw them, 10 riders picking their way slowly down the mountainside through the bog.
We were out for a walk in Batt's Estate near Pierce's Castle, a rocky outcrop 465m up in the Mournes near the Leitrim Lodge carpark.
As we watched, the group, which was well equipped and was riding carefully, came up the slope towards us and past hikers on the main path into the mountains.
The bikes were loud and their engines left a lingering smell, but they moved on quickly and silence soon returned to the hills around us.
But though their presence may have been short-lived, the people who farm the mountains and those who work to preserve this environment say this activity is unwanted and damaging.
There are signs up at the entrance to Batt's Estate, a patchwork of private land in the western Mournes, banning the use of quads and scramblers.
But a representative of the farmers who work the land there told me that they were "plagued" with quad and scrambler activity, especially at weekends.
He told me that he had gone to speak to several groups he'd spotted on the mountain. Some, he said, appeared to have no idea that they shouldn't be there, others could be verbally abusive.
"They feel they've as much right to be there as anyone else," he said.
But the reality for the farmers is different.
Firstly, there is the damage some riders cause to walls and gates trying to gain access. Then there is the problem of livestock, especially sheep, being frightened off their grazing grounds.
However, there is no suggestion that the group I saw were guilty of any of that.
If anything, the 10 riders I met were acting responsibly and riding carefully in single file.
But no matter how careful they were being, there's no doubt their bikes were causing significant damage to the heath land, which is an important habitat.
Erosion is already a big problem in the Mournes. The Mourne Heritage Trust is spending £160,000 upgrading routes through the mountains. That work is currently under way on the slopes of Slieve Binnian near the Ben Crom reservoir.
Most of that erosion has been caused by walkers over the years. But motorbikes can do much more damage.
In normal circumstances, water will drain evenly from the heath.
But when motorbikes cut through it, they leave a track in the bog which rain will use as a water course. Over time, that causes slippage in the bog which can leave large scars on the landscape.
The Mourne Heritage Trust has been working through the motorcycle clubs to raise awareness of the problems scrambling causes in the mountains.
But Matthew Bushby of the trust said it was still a big issue.
"They may come through in ones and twos or they may come through in bigger groups as you've seen," he said.
"Some seem to be quite well organised.
"It comes in fits and starts. Around Christmas for example, you may get a group coming out to do a run. We're trying to liaise with landowners, the Environment Agency and the police to try to build together a way of managing this.
"It's very difficult for landowners on their own to do something about this."
The eastern Mournes, where the main mountains like Slieve Donard, Bearnagh, Commedagh, Binnian, are all found, is an area of special scientific interest and has extra protection from the Environment Agency.
The western Mournes round Hen and Cock Mountain, Eagle Mountain and Shanlieve, where I saw the scramblers, is being considered for ASSI designation, but it hasn't happened yet.
The Mourne Heritage Trust favours talking to those who use the bikes about finding sites where it would be appropriate for them to enjoy their sport.
In the Republic, they've taken a much more hardline stance. Leader of the Green Party and former minister John Gormley, brought in legislation to ban scramblers and quads in 14 designated areas.
Anyone caught using one there, faces a 5,000 euro (£4,400) fine and a potential prison sentence.