Underground!

When you need to bury wiring, or handle any outdoor wiring it is best to call a master electrician. There are NEC - National Electric Codes covering all outdoor wiring you might need. 
​There are rules covering  new receptacles, conduits and outdoor cables, to outdoor lights. 

 With outdoor wiring, the primary safety concerns involve shielding against moisture and corrosion, preventing physical damage, and managing issues related to underground burial. With most residential outdoor wiring projects, the relevant code requirements pertain to installing outdoor receptacles and lighting fixtures, and to running wiring above and below ground. Where the official Code requirements call for "listed" equipment, this means that the products used must be authorized for the application by an approved testing agency, such as UL (formerly Underwriters Laboratories).  Source: The Spruce. ​