Some of them are still "live": unexploded. The bomb disposal squads come around ever so often and remove them. The only items that cause immediate action are poison gas shells, which might be leaking.
I found this large shell, about knee-height and appearing to be still live, at La Boiselle on the Somme. While I was taking the photograph a school party came by, from Yorkshire to judge by their accents, and the teacher in charge did the least intelligent thing I have ever come across: he walked up to the shell and kicked it! I was distinctly scared!
One tour-guide told us how children from another school party had found a live trench-mortar shell and had tried to take it home with them. Fortunately it was intercepted and seized by Dover Customs.The bomb disposal squad reported that it was extremely volatile and could have exploded at any moment!
Some of the relics of war are far larger: this concrete pill-box near Ypres, for example. We were told that it is actually upside down, having been turned completely over by a gigantic explosion.
This dugout, also in the Ypres area, was apparently where Adolf Hitler was once based.
Trenches, shell-holes and mine-craters still scar the landscape, but the two battlefields most likely to be visited by British tourists (that is, Ypres and the Somme) are quite different in character. Around Ypres, being low-lying and muddy, the trenches and craters are likely to be flooded with water,
wheres at the Somme, an area of rolling chalkland, they will generally be dry.