How to splice into existing underground rigid metal conduit below ground and hide junction box?
314.29 exception allows listed boxes and handhole enclosures shall be permitted where covered by gravel, light aggregate or noncohesive granulated soil if there location is identified and accessible for excavation.  I have used landscape border at the ground level to identify the buried location. This has passed inspection just make sure to use a quality listed splice kit as this will be in water at times. Also get some thwn (most thhn is dual listed thhn / thwn)  and put it in conduit 1 stick of pipe will get you there.