Ann: BP33 Diamond Drilling Results, page-65

If you read through the mine proposal for BP33, it actually lays it out pretty clearly.

There will be a surface cut, then a descent via a 6m by 6m corridor at an incline. The method of mining will be up hole retreat, without needing backfill. The geology around the resource supports this well (no need for costly backfill).

All in all the cost to mine is about the same as the open cut mines, on a cost per ton basis.

Additionally, in a recent Global lithium podcast episode where they had the LTR CEO on, he explained clearly why underground mining is the preferred method of mining. You have minimal distruption to the surface, and in the age of ESG this is a much preferred method compared to an open cut (where you damage the land, see what happened to Rio Tinto). Traditional owners are much less likely to oppose as you are not digging a giant hole in their lands, and you can easily return backfill and waste without causing issues.