North Stack Fog Signal is located on the headland a few miles north of South Stack Lighthouse.
A fog signal station was first established at North Stack around 1780. Initially, a bell sounded a warning. The signal was acquired by Trinity House in 1857 when two cannons were installed. The fog signal was used to assist South Stack Lighthouse, as the signal could not always be heard when the wind was blowing in the wrong direction. The fog guns fired every 4 minutes.
The site consisted of a number of buildings including a magazine with a curved roof used for storing the shells for the cannon fog signal. The magazine was built in 1861.
Two separate buildings were home to the fog signal keepers and their families.
In December 1944 a US Air Force B-24 Bomber crashed at the site, killing all eight crew on board. There is a memorial plaque in the Holyhead Breakwater Country Park on one of the propellers, which names the eight crew who were never recovered.
In the latter half of the 19th century, electric tannoy emitters replaced the fog cannons. The new fog signal stack contained 35 tannoys, but this was silenced in 1986 when a new signal was installed at South Stack lighthouse. Electrification arrived at North Stack in the 1950s.
The two cannons were thrown over the cliff when they were discontinued in the 1960s. One was later rescued by divers in 1984, and is now on display at Holyhead Breakwater Country Park.
The former magazine was later used as a bird observatory, and artist Phillipa Jacobs lived at the site for over 20 years.
North Stack is a popular spot for rock climbing.