Þingvellir became a national park in 1928 due to its historical importance, as well as the special tectonic and volcanic environment. The continental drift between the North American and Eurasian Plates can be clearly seen in the cracks or faults which traverse the region, the biggest one, Almannagjá, being a veritable canyon. This also causes the often-measurable earthquakes in the area.[7] Some of the rifts are full of surprisingly clear water. One, Nikulásargjá, is better known as Peningagjá (lit. "coin fissure"), as it is littered with coins at its bottom. After being bridged in 1907 for the arrival of King Frederick VIII of Denmark, visitors began to throw coins in the fissure, a tradition based on European legends. Þingvellir is situated on the northern shore of Þingvallavatn, the largest natural lake of Iceland. The river Öxará traverses the national park and forms a waterfall at the Almannagjá, called Öxarárfoss. On the lake's northern shore Silfra fissure is a popular dive and snorkel tour location. Together with the waterfall Gullfoss and the geysers of Haukadalur, Þingvellir is part of the most famous sights of Iceland, the Golden Circle. Þingvellir is a designated UNESCO World Heritage Site.