The Menai Strait is a narrow stretch of shallow tidal water about 25 km long, which separates the island of Anglesey from the mainland of Wales. It varies in width from 400 metres from Fort Belan to Abermenai Point to 1,100 metres from Traeth Gwyllt to Caernarfon Castle. It then narrows to 500 metres in the middle reaches and then it broadens again. At Bangor, Garth Pier, it is 900 metres wide. It then widens out, and the distance from Puffin Island to Penmaenmawr is about 8 kilometres.
The differential tides at the two ends of the strait cause very strong currents to flow in both directions through the strait at different times, creating dangerous conditions. One of the most dangerous areas of the strait is known as the Swellies between the two bridges. Here, rocks near the surface cause over-falls and local whirlpools, which can be of considerable danger in themselves and cause small boats to founder on the rocks. This was the site of the loss of the training ship HMS Conway in 1953. Entering the strait at the Caernarfon end is also hazardous because of the frequently shifting sand banks that make up Caernarfon bar.