I believe the additional permit that you are asking about is what's called a Plan of Operation, but based on what you mentioned you won't need one for casual use. Is the land for sure BLM or is it National Forest? Sometimes the Forest Service requires a additional plan of operation even for casual use (you can call the local National Forest office and ask), but if it's just BLM land then you do not need a plan of operation for casual use. A plan of operation is usually only necessary if you would like to use machinery, generator powered tools or blast and after you hold the claim you apply for plan of operation and then the BLM will do a site visit and they usually make you put down a land reclamation bond of at least a few thousand dollars to make sure you remediate the disturbances you create when you are done. They also control how much earth you are allowed to disturb every year when you are using machinery. My understanding is that you are not responsible for remediating pre-existing tunnels, pits, disturbances, buildings, etc... but you can contact the state BLM office and ask. The BLM will often suggest you keep out of old tunnels on your claim for safety reasons, but if your just going into old tunnels on your claim and doing minor collecting along the existing walls with hand tools that is fine, but if your attempting to full on tunnel then you would need a plan of operation, and my understanding is that underground mining on public lands comes with a lot of red tape like MSHA training, certification and inspections. Note that casual use claim holders can NOT do any of the following on their claim: store any tools, erect sheds, put up fences or signs, charge for the public to do fee digs, & live on your claim. I was informed you are allowed to camp on your casual use claim for 14 days every quarter by the California BLM office. I've found that the hardest form to find online is the first one you need to actually file the claim called the 'Lode Mining Claim Notice,' all of the other forms you need can easily be found on the BLM website. The BLM 'year' runs from Sept 1st on, so if you file before Sept 1st 2021 you will need to fill out the Lode Mining Claim Notice, placing it in a jar on the claim at the discovery marker, getting it recorded by your county, and then sending the recorded copy to your state BLM office, and then placing a copies of the stamped & recorded form you get back from the BLM on your claim at all of the markers. Then before Sept 1 2021 you will need to start turning in paper work for next year, which for the first year is a Notice of Intent to Hold form. From then on you can either do the 1) annual Maintenance Fee Waiver Certification (for small miners owning <10 claims) + submitting a county recorded Affidavit of Annual Assessment Work (aka proof of labor form) or option 2) you can just pay the Annual Maintenance fee.