Ginormous snails

My city garden has attracted a family of oversized snails. The adults are around 15cm long when outstretched under their shell (see photo). Their offspring – I have seen 2 (but I accidentally stood on one in the dark of night) are around a third of the size of the adults. They are not at all shy and are often seen moving around even in the most unusual places, like along the wooden trusses of the roof on the veranda. They move surprisingly fast for snails. Is someone able to identify them?

Pat Liebetrau (