Areas that were difficult to access or infrequently visited during WW2 may have a higher risk of unexploded ordnance. Bombs that were dropped in rivers and harbours are more likely to have escaped notice if they did not explode and are found reasonably frequently, as these examples in London Docklands, the River Thames, the River Mersey, Portsmouth, Dover, Southend, Sussex, Devon, Herne Bay and Bristol demonstrate. If a construction project is located on a drained marsh or former water course, in an area that was subject to bombing, a detailed risk assessment will be required.
Rainham Marsh, east of London (pictured), was used a bombing decoy and firing range, so had a military presence during WW2. Even so, it was only in 1984 that a Hurricane fighter plane and its pilot, that had been lost during the Battle of Britain, were recovered.
Quarries were sometimes used as bomb stores and bomb cemeteries, the latter being a place that defuzed bombs which still had explosive in them were taken for final disposal. Redundant quarries have often been used as landfill sites and it is possible that ordnance could remain in these locations, as the photo shows.
Due to the high volume of material that is extracted from quarries, should there be UXO at a site that is currently a quarry, there is an increased probability that it will be discovered. This report on a bomb that was found in Portland also describes the disruption experienced by the local community during the process to make it safe.
This link provides information about a post-WW2 fatality at Upminster bomb cemetery, which is now a landscaped rubbish dump. These links are for media reports on the potential presence of ordnance at a former quarry and bomb store in north Wales, WW1 ordnance discovered near Basingstoke and a WW2 bomb in Hainault.
It is worth noting that an incident was recorded on a construction site at the former Normanton Quarry, where a banks-man was killed when discarded non-military detonators were accidentally struck by an excavator.
It is still common for people to retain memento to remind them of life's experiences. When these items are explosive ordnance, they will continue to pose a hazard to the keeper and those nearby. These items can be found anywhere, they are sometimes dumped illegally and have been discovered when clearing the houses of deceased people. These links are to news reports on the discovery of live WW1 ordnance in Somerset and a dry stone wall in Devon; the photo shows a WW2 German 10.5mm anti-aircraft round being used as a doorstop, that was discovered in the UK.
The recent rise the popularity of magnet fishing in watercourses has meant that it has become more common for UXO to be discovered by the unwary. The risk that these items continue to pose should not be underestimated. This link is to a news report in Leicestershire, following the discovery of a grenade.