Tunnels & "Caves" in Napa Valley, California?

Under Speaker Pelosi's vineyard possibly?

What open source data could reveal this?

-Building permits

-Changes in county tax assessment

In some counties, the tax assessor documents change in the assessment photographically

The person seeking a building permit generally has to submit plans and diagrams for approval

These can be found in the county tax records

Some counties let you search tax and property records for free

Some provide access to these building-permit-related documents online

Avenues for digging?