The item is headed 'Concern about a probable chalk mine under a railway tunnel at Charlton, southeast London " and relates to an enquiry from Network Rail concerning cavities encountered on the North Kent railway line. They say "the tunnel had been driven between 1847 and 1849 by John Brogden (junior) [1823-1867]. This line was opened to Charlton Station on 30 July 1849, but the next section to Woolwich Dockyard Station (opened 1 November the same year) was evidently slightly delayed by the tunnelling and the unexpected cavity".
The article is by Paul Sowan who is coming to speak to GIHS again on 19th November. But otherwise read Subterranea for the whole story.