Banc du Geyser (also Banc du Geysir) is a mostly submerged reef in the Mozambique Channel's northeastern part, 125 km (78 mi) northeast from Mayotte, 112 km (70 mi) southwest of the Glorioso Islands, and 200 km (124 mi) off the northwestern coast of Madagascar.

The Banc is a dangerous oval-shaped reef 8 km (5 mi) long and 5 km (3 mi) wide that becomes exposed only at low tides, with the exception of some rock formations in the southern part of the reef. The rocks are generally 1.5 to 3 meters (5 to 10 feet) in height; the largest is South Rock, with a height of 8 meters (26 feet), similar to a boat under sail. In the eastern part of the reef there are some sandy cays, 1 to 3 meters (3 to 10 feet) in height covered with grass and small bushes. The entrance into the central lagoon is possible from a south-southeastern direction. There is an abundance of sea birds, and the cays are covered in tons of guano.

The Geysir Reef was first known by Arab sailors around the year 700, and was shown on some navigation-charts dated around 800. Around 1650 the reef was shown on Spanish maps as Arecife de Santo Antonio. The current name was given on 23 December 1678, when the British vessel Geysir ran on the reef.

Banc Du is a prominent, fairly flat-topped southwards-projecting promontory of Foel Eryr, at about 334m OD at the west end of the Preseli Hills in north Pembrokeshire, Wales. First spotted in July 1990 during an aerial photographic survey of the region, this hilltop enclosure consists of two non-concentric and incomplete earthworks that define a roughly oval area around the hilltop, amidst a series of cultivation features and abandoned trackways.

There is a crag-line on the south-east side and steep slopes to the south and west. It overlooks the source of the Afon Syfynwy to the south-east, and afford extensive views to the south and west. Neither upland not lowland, Banc Du occupies a curious position between enclosed fields and open moor, clearly cultivated in the past, but now maintained as improved pasture for grazing sheep.

Surveys suggest three main periods of activity across the hill. The most recent comprises extensive ridge and furrow cultivation probably associated with a now-abandoned, putatively medieval or later, long house settlement or small hamlet downslope to the north-west. Earlier than this are two curved trackways that seem to provide access to a single long-house dwelling or structure. It is currently unclear whether these are contemporary or successive.