One of the difficult technical challenges to be considered is the fact that the strait is near the Sunda Trench in one of the world's most dangerous earthquake zones. Sumatra is frequently rocked by significant tremors; more than 170,000 people in Aceh in northern Sumatra were killed when a 9.0-magnitude quake in December 2004 triggered a tsunami. Many active volcanoes lie in the area, including Krakatoa only 40 km away. The best known eruption of Krakatau in 1883 culminated in a series of massive explosions that killed tens of thousands of people.