Like many of the islands that make up Indonesia, Pulau Weh has volcanic origins and still hosts and active vent. There are three distinct fields of small vents crossing the mountains in the center of the island in a line from the ferry harbor in the southeast to the base of the bay where Gapang and Iboih beaches are located in the northwest. There is also a fourth vent field underwater south of Gapang beach. This is a very interesting place to dive.
The scarred landscape of Weh's volcanic area.
The vent fields on land are like big scars on the landscape. The sulfurous fumes from the vents kills off all plants and trees in the area. The vents are mostly quite small and spit out a stream of hot gases, giving the area that distinctive rotten-egg odor. There are only a couple of places where water is also bubbling out of the vents, but it was the end of the dry season when I visited. If you come earlier in the dry season, you might see a lot more water flowing, as it's evident from the landscape that there are times when a lot more water is flowing through the area.
There is a trail that leads from the road to the vent area, but the roads do not seem to be very well signed for the place, so if you're driving on your own you may need a good map.