It's estimated that there's about 17,000 tonnes of bombs that were dropped on London in WWII, and the rule of thumb is that about 10% failed to explode....

Freeman Simon Cooke's 2013 article, including his engineering input to this project, is at wcomc.org/sept30-1 :  accompanied with astonishing Crossrail statistics, wartime underground maps and pics, at http://www.bbc.co.uk/news/magazine-23518137.

Crossrail 2017 Update....

Since the start of construction in 2013, more than 10,000 objects from prehistory to the present day have been unearthed, giving archaeelogists significant insight into our past - from early geological changes to the habits of Roman London, the DNA of the Great Plague and Black Death, and beyond....

Photos / maps / events at -  http://www.crossrail.co.uk/news/quarterly-crossrail-update/

Crossrail Tunnel archaeology dig...