As a mining engineer that has designed a number of mines in various locations around the world, at a depth of ~400m and with he thickness reported, it would never be viable to move underground. If the price of lithium doubles it still wouldn't be feasible. While it does add to the JORC compliant reporting, it just isn't feasible.
One person mentioned "new mining technology", there is nothing that can magically transport you ~400m underground. Depending on the lithology of the area the cost of developing a decline is between $7,500 - $12,500 per metre. Assuming the decline was started at the base of the pit, then a 300m deep decline would cost up to $37 million (decline would need to be graded at 10%). if you have gold grades of 8g/t it might be worth it, but not for a thin layer of pegmatites.
While 350m isn't deep for a typical surface mine, remember that the majority of surface mines have the mineralisation continuing down (it doesn't just stop), as this announcement shows, the mineralisation doesn't continue at depth.
I think the more interesting information from the release is that pegmatites continue to the East and West at a depth less than 100m. Not trying to down ramp, i do hold, just being realistic.
