Holyhead Breakwater.

At 1.7 miles long it is the longest in the UK.
1300 men took part in the building of it, over 28 years from 1845 until 1873 and during that time 40 men died in its construction.
Mainly rock from Holyhead mountain was used and dressed with limestone from a quarry in Moelfre. A broad gauge railway was built to bring the stone down the mountain and later this was used to transport the bricks produced at the quarry.  
It is a listed structure as is the lighthouse at the end, which retains all the character of a traditional Victorian lighthouse. It has been fully automated since 1961.