I'm building a propane fire pit in my backyard patio with a pea stone surface. I want to have the LP tank located 12 feet away from the fire, and I don't want to trip over the line. What can I do for an underground connection for a small application like this? For the burner, I'll be buying a kit from FirePitsDirect.com.
Copper, steel and polyethylene are typically used for propane & natural gas - check your local code. I'd probably go with black iron pipe - it's cheap, tough, readily available, and relatively easy to work with. Dope the connections with pipe dope approved for gas, and pressure test the assembly before putting it into service. It should be buried at least 12" deep.
Note that the CSST (Corrugated Stainless Steel Tube) flexible gas line typically requires a conduit for direct burial.