As far as I know, the gardens were laid over the graveyard.
Which is probably the reason why it's always so well looked after.



One of the theories put about was that the graveyard was cleared for the building of St George's Hall, but that's wrong on a number of counts.
St George's Hall was built on the site of buildings, including the Infirmary.
(The graveyard was behind the site, where the gardens are today).
St George's Hall was completed before the dates on some of the graves.

The real reason for the mass grave will probably always be a mystery, but my personal feeling is they were plague victims who were buried in what would have been the countryside.
There again the cholera epidemic was a good ten years before the dates on some of the graves.