Don't bury an extension cord — even temporarily.

No. It’s not safe. A waterproof cover doesn’t change the fact that extension cords aren’t rated for direct burial.

The problem is moisture will eventually work its way into the connection points and the cord itself. Even a “waterproof” cover isn’t airtight underground. Dirt holds moisture, pressure forces water past seals, and now you’ve got a corroded live connection buried in your yard. That’s a fire risk and a shock hazard.

Building codes exist for a reason. Proper underground landscape lighting uses UF (underground feeder) cable or direct-burial-rated wire with waterproof splice kits. That stuff is designed to handle earth contact, temperature changes, and ground pressure.

If this is a temporary setup for a weekend party, run the cord on top of the ground and tape down the connections with electrical tape. If it’s going to be in the ground longer than that, do it right.