The Stanley Embankment or "cob", as it is called by local people, was built by Thomas Telford, whose 250th anniversary was celebrated on Anglesey this year.
It links Holyhead and Holy island with the rest of Anglesey.
The embankment carries the local traffic from Valley and the surrounding villages, including Amlwch in the North, into Holyhead on the old A5 trunk road which starts in London.
Prior to building a new dual carriageway trunk road across the island, the A5 would meander its way through the middle of the island, passing through the small villages.
This meant the 25 mile journey on a single carriageway from Holyhead to Bangor, on the mainland, could easily take about 40 minutes.
With the new A55 trunk road by-passing the villages of Llanfairpwll, Gaerwen, Pentre Berw, Gwalchmai, Bryngwran, Caergeiliog and Valley, the embankment has also seen a fall in traffic volume.
You can see the direction of the A5 and A55 trunk roads to Holyhead on the map here.
Now yopu can keep up with the latest news, articles and video clips by getting your free copy of the Anglesey Today Newsletter. Just complete the simple form here; it's as easy as 1-2-3!
At the Holyhead end of the embankment you can visit a former turnpike or toll-house, where tolls (fees) were collected from travellers on their journey across the island.
Today the building has been sensitively renovated and is run as a tea shop, with a garden and car park at the rear. It is also outside the entrance to the Penrhos Country Park.
As you cross the embankment you are in effect walking along the Anglesey Coastal Path, and when you reach the Holyhead side you just fork right and follow the blue and yellow signs into Penrhos Country Park.
At the middle of the embankment is a channel through which the tidal water flows in and out of the inland sea. This is a spot popular with kayak and canow enthusiasts.
Sometimes you see them practicing their moves and stamina by paddling against the currents.
There is a high wall behind which is the Holyhead to London Rail link. Running parallel with the Stanley Embankment is a new structure which carries the new A55 dual carriageway into Holyhead.
The bulk of the traffic uses this new "shadow toll" road which was built using the private finance intitiative. This includes the trucks and cars heading for Ireland via the Holyhead Dublin ferry.