Some prospecting and exploratory work on the Grotto, also known as Larrabee's Prospect, was first documented in 1925. It was reported in 1927 that earlier prospecting revealed the presence of a large cave in limestone at the bottom of which a small pocket of galena was exposed. A 58-metre crosscut was driven and 15 metres of drifting done. A number of short diamond-drill holes were also completed in 1927. The next recorded work takes place in 1972 when Grotto Silver Mines Ltd. collected 200 soil samples, drilled one diamond-drill hole totalling 85 metres and did some trenching and stripping. In 1975, Cominco Ltd., for Grotto Silver Mines Ltd., completed four diamond-drill holes totalling 272 metres as well as some rotary drilling, mapping and road construction. Grotto Silver Mines did 60 metres of trenching in 1976, 12 metres of drifting in 1977 and 91 metres of diamond drilling in 1978. In 1979 Grotto completed 106 metres of diamond drilling, took 100 soil samples and completed some trenching.

Disseminated pyrite, galena, sphalerite and smithsonite occur in fractures and carbonate breccia in dolomites of the Cambrian Jubilee and Ordovician to Silurian Beaverfoot Formation.

It was mentioned in 1927 that 550 metres to the west of the Grotto cave, a vertical sheared fracture is slightly mineralized with quartz and galena across a width of 1.8 metres. A sample taken across 60 centimetres yielded 0.69 gram per tonne gold, 92.58 grams per tonne silver, 4.7 per cent lead and 0.7 per cent zinc (Minister of Mines Annual Report 1927, page 265).