There were two rocket systems used by the Germans during WW2:
The German name translates to vengance or retribution weapon.
Known to the British as flying bombs, pilotless aircraft, doodlebugs and buzz bombs, they were early cruise missiles with a range of 250km, carrying a 850kg explosive payload. Over 9500 were fired at south east England from June 1944 to March 1945, although many were destroyed prior to crossing the English Channel. It has been estimated that over 6000 people died due to these weapons.
These were the world's first guided ballistic missiles, having a range of up to 320km and carrying a 1000kg explosive payload. Over 1400 rockets were fired at England from September 1944 to March 1945 and over 2500 people died in London due to them. Whilst many exploded harmlessly in open countryside, a single rocket in New Cross killed 160 people.
The V1 rockets were constructed from relatively thin metal, so if they did not explode on impact with the ground as intended, could disintegrate. Their triple fuzing mechanism was effective and included a timer as a back up, so relatively few failed to detonate. A rare example of a V1 that has been recently recovered was at Marsh Wall in London's Docklands in 2007. For additional information on that incident, click here.
By 1944, the civil defence system was well established and detailed records showing the locations of V1 strikes were maintained. It is worth noting that unexploded V1 that were not identified during WW2 could not have been recorded on these maps. It is these unidentified bombs that now pose a threat.
V2 rockets were propelled upwards for about a minute, typically reaching a height of 80-90km, then fall to Earth, a process which limited their accuracy. Additionally, V2 rockets could not be fuzed to explode at or just above ground level, but exploded underground, which reduced their blast effect.
Detailed records of these attacks were maintained. These rockets were much larger than most aerial delivered high explosive and incendiary bombs, so the possibility of an unexploded V2 remaining undiscovered after impact is remote. It is very unlikely that a V2 rocket would be discovered on a construction site.