On several bridges above the Mass Pike, a series of locks dots the chain link fence. These locks come in all sizes. Some have rusted shackles, and others are shiny and new. Many are red. More are heart shaped. A few even bear engravings, “Love You Forever.”

Around the world, these are known as love locks, a popular tradition in scenic cities. Couples write their names on the lock, put it on a bridge and throw away the key, to symbolize that their love is locked forever.

Usually, love locks are found on bridges above rivers, both for the scenery and so that the key is truly gone forever when it is thrown into the water. In Boston, the locks can be found on the Mass Ave overpass by the Hynes Convention Center T station, the Brookline Avenue overpass near Fenway Park, and sparsely on other bridges across the Pike, like those near Boston University.

Love locks are part of a global tradition which seems to have started in 2006, after an Italian romance novel featured the Roman lovers writing their names on the padlock and throwing the key into the canal.

They became most popular, however, in Paris, on the Pont des Arts above the Seine River. In stark contrast to the fences of the Mass Pike overpasses, the locks on this bridge were so dense that it was impossible to see the fence itself. Couples from around the world would travel to the City of Love to memorialize their bond on the bridge and throw the key into the river, as proof their love could not be undone.

But the symbolic locks still have real world consequences. In 2014, a section of the fencing on the Pont des Arts collapsed under the weight of hundreds of locks. Since then, the fences have been replaced by glass panes, to preserve the structural integrity of the bridge.

With similar concerns, MassDOT regularly purges love locks from the numerous Mass Pike overpasses that house them. The department said it was aware of the locks and periodically removes them when their weight becomes a hazard to the fence.

It did not specify how it determines when the locks are hazardous but has likely not purged the Mass Ave overpass in at least two years. One red heart shaped lock on the fence reads in white letters, “Lisa & Josh, Locked in Love Forever, 2023.”

After every purge, the locks slowly trickle back. Some are simple hardware store padlocks, with lovers’ names scrawled in permanent marker. Others are golden heart shaped locks, sold in pairs because they are meant to be intertwined. If you look closely the next time you cross the overpass, you will see just how varied and beautiful these symbols of love can be.