Dettifoss

Dettifoss is found in the northeast part of Iceland between Myvatn and Egilsstaðir, north of Highway 1. There are two roads which access the falls, one on either side of the suspension bridge along Highway 1 which spans the Jökulsá á Fjöllum, which itself is found 130km west of Egilsstaðir, or 35km east of the Reykjahið (Myvatn). Both roads are signed for Dettifoss.
To reach the east side (easier access) of the falls, take Route 864 - which branches from Highway 1 just east of the bridge - north from Highway 1 for 32km, then turn left where a sign points to Dettifoss, the parking area will be found another 0.75km down the road. A trail then leads about one-third of a kilometer down to the first viewpoint, and continues further to the very brink of the falls. Note that Route 864 is gravel and as of August 2014 was in pretty rough condition (2wd vehicles are allowed on this road, it just needs to be driven slowly).
To reach the west side of the falls, take Route 862 - which branches from Highway 1 about 9.5km west of the bridge - north from Highway 1 for 22km - then turn right where a sign points to Dettifoss, the parking area will be found just over 3km further. A trail then leads down to the rim of the gorge opposite the falls in another kilometer. Note that Route 862 was formerly classified as an F-Road (4wd high clearance only), but that does not appear to be the case any longer.Dettifoss is the largest waterfall in the Jökulsá á Fjöllum (River), as well as one of the largest (in terms of scale) in Iceland. Situated on the second step of the Jokulsargljufur - Selfoss occupies the first step just upstream - the river rumbles across a broad exposure of columnar basalt, stretching to a width of about 500 feet, and then hurtles 167 feet in a massive, immensely powerful curtain of water into the canyon below. As the river sources in several huge lobes of the massive Vatnajökull, and flows through areas of active volcanism, there is almost always a high volume of silt in the water. This causes the river to assume a color ranging from slightly gray to straight up chocolate milk during the warmest summer months. As the river plunges over the precipice the color of the water actually accentuates the power of the falls.
Though the Jökulsá á Fjöllum is (apparently) on average only the third largest river by volume in Iceland, because of the height and width of the falls, Dettifoss is often considered to be the most powerful waterfall in Europe. While it may be difficult to quantify such a claim, it's hard to discount such a statement given that there are very few other waterfalls of similar or greater size on rivers of similar volume elsewhere in Europe. Signs at the falls indicate that the river's discharge averages about 14,125 cubic feet per second in the summer months, but this contradicts the river gauge measurements we've been able to find which shows the flow to vary from about 3700 cubic feet per second in the colder months to about 9,500 cubic feet per second in the warmest months of the summer, with peak flow hitting over 20,000 cfs periodically.
In addition to the wildly varying volume of the river, the Jökulsá á Fjöllum has been prone to several immense Jokulhaups, or Glacial Outburst Floods, in the past. Often these floods have been instigated by volcanic activity in the Barðarbunga caldera, which lies beneath the glacier that sources the river. The canyon which forms Dettifoss was likely formed by several such floods repeatedly scouring the bedrock. Some analysis suggests that in order to account for boulders which have been deposited in certain locations throughout the canyon that the volume of water involved in such floods would have had to have been greater than the volume of the Amazon River! Obviously such an event would be short lived however. This hypothesis has achieved recent attention due to the present volcanic activity at Barðarbunga - which in fact began the day we arrived in Iceland, and two days after we visited Dettifoss the roads accessing the falls were closed off in precaution (the volcano didn't begin erupting until after we had left the country). Thus far there has been no indication of an imminent flood, however should one occur as a result of the activity, there is potential for Dettifoss to be permanently altered.
Movie goers may recognize Dettifoss from the opening scene in Ridley Scott's Sci-fi film Prometheus.