Better act fast because the WWE has collected the soil and put it up for sale -- and we're told it's already in super-high demand!!

If ya missed it ... 55-year-old Undertaker beat Styles in an epic "Boneyard Match" at Mania back on April 4 -- which featured the two wrestling stars fighting it out in a cemetery.

Now, to commemorate the tilt, WWE is selling real dirt from the night ... and the piece is actually pretty sick.

The memorabilia features a plaque with photos from the evening as well as a capsule full of the historic Earth that Styles was ultimately buried under.

WWE is listing the items for $74.99 a pop ... and is limiting quantities to just 536 total.

One source tells us ... "Demand is strong and it's already a top-selling item" on the official WWE online shop.

Of course, while it is just dirt ... WWE mementos tend to age pretty well -- some of the game-used items from iconic wrestlers in the past have sold for thousands!!