One of the most hazardous areas is the Swellies (Pwll Ceris), between the two bridges, where rocks near the surface cause over-falls and local whirlpools.

The present day channel is a result of glacial erosion of the bedrock along a line of weakness associated with the Menai Strait fault system.

During a series of Pleistocene glaciations a succession of ice-sheets moved from northeast to southwest across Anglesey and neighbouring Gwynedd scouring the underlying rock; the grain of which also runs in the same direction.

The result was a series of linear bedrock hollows across the region, the deepest of which was flooded by the sea as world ocean levels rose at the end of the last ice age (c. 10,000 BC).

"[3] According to Heimskringla, the 11th century Norse-Gael ruler Echmarcach mac Ragnaill plundered in Wales with his friend, the Viking Guttorm Gunnhildsson.

The most recent service appears to have been Feb-2021[1], since when the vessel has been taken to dry dock for essential repair work The tidal effects observed along the banks of the strait can be confusing.

[4] Because the strait has such unusual tidal conditions, coupled with very low wave heights because of its sheltered position, it presents a unique and diverse benthic ecology.

The existence of this unique ecology was a major factor in the establishment of Bangor University's School of Ocean Sciences at Menai Bridge, as well as its status as a special area of conservation with marine components.

[citation needed] The same unique ecology and geomorphology has let to a number of designations of SSSIs along the strait including Glannau Porthaethwy, the ivy–oak–ash woodland on the southern shore (Coedydd Afon Menai) and Lavan Sands (Welsh: Traeth Lafan).

However, on 14 February 2023, the Welsh Government announced that the project would not go ahead, citing efforts to reduce car usage, its environmental impact and it being a "blot" on the landscape.

[14] Lee Waters, deputy minister for climate change, later stated the crossing could be considered again as part of a wider review into the infrastructure of North Wales, rather than individually.