Put a fence wall under the map. If you don't know how to do that, read the "Merging Movable Objects With Map Geometry" guide.
I think you just sink a fence wall underground, then save while still holding it. Then go beneath by sinking under a doublebox. Grab the fence wall and place it where you want the box to go, then save again while holding. Then I think you could probably go up in a new game, place the box you want to merge, merge it a bit, go underneath, and then pull it down and rest it on top of the fence wall. Did this help you?
no, you are all wrong. The most effective way is to place the box down then secure it on all sides, Geo-merge it normally 2 times. on the third time though, you must grab the box then without letting go save and quit, start up the game again. it should be about 70% in the ground do the same door geo-merge again, save and quit without letting go, then start it up and it will automatically be magically in the ground. If you need help send me an invite tomorrow. at 3:00PM PST to EGS Berserk