Pinpoint the target and then go after it with a garden claw -- oh yeah, by the way, pick digging spots where a bombed out minefield motif, would blend in with the surroundings. Lots easier if you are in a pasture or old site where landscape does not matter. Helps even more if owner, YOU, do not care. Try pre-construction sites, where things are going to get torn up later anyway, sidewalk tearouts, home sites, buildings, street right-of-ways etc.
Otherwise -- Plan A: Use Pinpointer (probe style or block studfinder in metal mode) and screwdriver or metal probe or needlenose Fishing pliers (long reach)
Plan B: Cut 3-sided plug (Sharpened Putty knife, Replacement Metal Plane Blade with Handle (Home Depot $5-Tools Section), or Garden Digging Tool)
Plan C: Ground Pulverizer, like weeding tool or garden claw to break up soil and fibre planting trowell (HD Garden Dept ($1-$2) -or- Mantis tiller works OK, but tends to scratch up things a bit. If you want to search an area you can do some real damage with a roto-tiller .. Troy Built rear-tine units are the best.
Plan D: Ace Hardware -- Nifty device Bulb Planter ($20) and Rechargeable Drill -or- one/two man post hole digger.
Plan E: Bobcat with trencher or backhoe.
Plan F: Build a drop down single blade or shovel unit that you can install in the trailer hitch receiver and then you can really go at it and tear up the field.
Just remember -- "Progress is our most important product."