OK I've now seen from elsewhere that this is hole in a brick arch near Bricklayers Arms rather than in the ground so my theory from my last post isn't how this one happened. Seems that questions have to be asked about bridge inspection and maintenance regime. I would suspect that this is the...
Whist noticing your cycnicism I'd say a) is quite likely as it does also happen with roads. Usual sequence of events is:
Underground water pipe springs a leak, no-one notices
Leak gets bigger, water flow starts to create cavity a short distance underground, no-one notices
Cavity gets bigger and...