[1] South of the mountain towards the coast is the Hellisheiði geothermal area to its south-west, and the town of Hveragerði on Iceland's main ringroad, Route 1 about 45 km (28 mi) east of Reykjavík.

[5] This seismic activity is significant not only because of human impact but also because the earthquakes are grouped along faults striking north–south or east–west, but not north to north-east, as might be supposed based on the surface geological structure and that the rift zone is deep as 3–9 km (1.9–5.6 mi) locally.

[6] Lake Thingvallavatn, occupies the proximal graben, and has at its southern end lava flows from the mountain and has hosted explosive eruptions historically.

[13] [a] There was either hydrothermal fluid or magma increase at a depth of approximately 5–7 km (3.1–4.3 mi) within the inferred brittle-ductile transition zone of the area centred at Ölkelduháls, between Hengill and Hrómundartindur to its east during 2017 and 2018.

[16] For example, in a folk tale collected and published by Jón Árnason in 1862, a young farmer is said to have killed the sleeping troll woman Jóra [ˈjouːra] while she lay in wait for innocent wanderers or horsemen on the trail over Dyrafjöll [ˈtɪːraˌfjœtl̥] north of Hengill.

[17] In another folk tale, according to some people, a woman's body found in the area was believed to be that of Halla, the wife of the famous Icelandic outlaw Fjalla-Eyvindur.