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Opened on 5 March 1850, the original Britannia Bridge (Pont Britannia in Welsh) was designed by Robert Stephenson as a railway bridge, consisting of parallel wrought iron tubes supported by the stone towers.

On 23 May 1970, a fire broke out in the tube at the mainland end, spreading across the whole bridge and making the tubes unsafe. It was subsequently rebuilt in its current form, as a two-decked arched bridge, retaining the original piers. The lower deck, carrying the railway (now single track), was opened on 30 January 1972, although the upper deck, carrying a wide single carriageway A55, did not open until 1980.

Grade II listed (and retaining the status, despite the rebuilding, because of the bridge piers) - see Link.

The Menai Strait (Welsh: Afon Menai, the "River Menai") is a narrow stretch of shallow tidal water about 25 km long, which separates the island of Anglesey from the mainland of Wales.
The strait is bridged in two places – the main A5 road is carried over the strait by Thomas Telford's suspension bridge, the first of its kind, opened in January 1826, and adjacent to this is Robert Stephenson's 1850 Britannia Tubular Bridge. Between the two bridge crossings there is a small island in the middle of the strait, Ynys Gorad Goch, on which are built a house and outbuildings and around which are the significant remains of fish traps, no longer used.
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 (or Swillies – Welsh Pwll Ceris) 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.
Wikipedia: Link