Geysers are usually surrounded by land and they spout hot water from underneath which happens as a result of the surface water touching hot rocks very deep underground. However, there are cases when geysers form due to particular hydrogeological conditions such as the one in Reykholt, Iceland. The geyser is surrounded by a river.
Iceland has many geysers but none is stranger than Vellir, also known as Árhver, because it is located smack in the middle of a flowing river. Vellir consists of a cone of cemented clay and gravel which can be clearly seen when the water level in the Reykjadalsá river is low, but at high water levels, the cone usually remains submerged.
(Image credit: Wikimedia Commons)