Mt Hengill is a 803 m high hyaloclastite and diorite mountain (possibly a canted table mountain) at the roots of the Reykjanes Peninsula in the southwestern corner of Iceland.  It is very prominent from the main highway between the capital area and the Southwestern Lowlands.  The mountain is most likely a part of a central volcano as the widespread thermal activity suggests.  Its highest point is called Skeggi.

The rhyolite mountain Sleggja extends to the south from Mt Hengill and the three valleys to the south are Innstidalur, Middalur and Fremstidalur.  In Valley Innstidalur is a large natural steam vent and nearby is a mineral spring.  Legend has it, that outlaws spent some time in the valleys in the past.  A cave with proof of human habitation was discovered in a large hyaloclastite rock just northeast of the steam vent.