The very visible cliff workings of the coves south of Porthmellin/Mullion harbour are often attributed to that of Wheal Unity Mine about half a mile inland, however this is not the case.
The main evidence for these steatite/soapstone workings are eluded to in 1752, when a lease of the property was granted to Nicholas Crisp and John Sanders - a jeweller and a potter - of London. The domain included the sea cliff between "a rock called Porth Pig" and "Pednankea". The indenture records the premiere on the North and latter on the South, however a location of a "Pednankea" is not know to be recorded south of the Harbour. The site of this place is likely "Pedn y ke" (which is pronounced "Pedn an key") suggesting that the cardinal orientations have been misrecorded.
The site of "a rock called port pig" is also in contention. Porth Pyg cove is recorded by the ordnance survey, however given its meaning in cornish of "Cove Point", it likely refers to what is now called the Vro Rock.
The tunnels visible from the harbour wall are accessible at low tide by the great cave at Porthmellin. They appear to be quite spacious and would suggest workings of a more modern origin. Further cliff quarries exist in Porth Pyg cove and adjacent the Vro Cove which offer more likelihood of being of this earlier pedigree.
Data courtesy of Ben Sum, Helston (19/11/18).
RIC = Royal Institute of Cornwall, Royal Cornwall Museum, Truro
[RIC] Indenture, Mary Vere of Lanhydrock to Crisp & Sanders, Goon Vean Croft, 1752