USE-2 solar 8 AWG electrical wire
|Please call for pricing over 500'
has a flexible rubber insulation suitable for direct burial applications. That means it can be buried underground without conduit. It's also suited for solar applications where a sunlight resistant approval
If you're looking for something less expensive you can use THHN THWN-2
which is NOT suited for direct burial. It's a very common electrical wire
used indoor, outdoors and in conduit applications.