Over the last 100 years of military flying, many planes did not land safely. The most significant risk is posed by bombers that crashed on the outward leg of a raid, prior to releasing their payload. In the UK, for Allied aircraft, this was more likely to be because of a technical fault, however for German (and sometimes Italian) aircraft, they were often shot down. It was estimated that the Luftwaffe lost 1014 bombers during the Battle of Britain alone. Whilst many aircraft were recovered at ground level (as indicated by the photo of a Dornier Do 17) others penetrated deep into ground.
Follow the link for a media report describing the recovery of a Junkers 88, in which a burnt out 500kg high explosive bomb was discovered near to Southport. It should be noted that some aircraft crash sites are considered war graves and their disturbance is prohibited.