Hvannárgil is a breathtaking canyon in the Þórsmörk area, at the bottom of which the Hvanná River makes its way wildly and can be a pain for drivers and pedestrians who wade through it.
Hvannárgil is an alternative, secondary, option to the Fimmvörðuháls trek, which takes you from the Þórsmörk area up to the upper almost to the upper plains between the Mýrdalsjökull and Dyjafjallajökull glaciers.
There are several spots in the lower part of the canyon that are pretty close to the cliff. Be careful where and how you step.
Hvannárgil is also a beautiful day trek option, where you either hike up the canyon and then take the same route down, or follow the Fimmvörðuháls trail up into the mountains and follow the Hvannárgil canyon back to the Básar area. But you should know that in the lower part of Hvannárgil Canyon, the road runs really close to the canyon rim.