The deep grass left and right of the regular-length rough at Erin Hills, site of the 2017 U.S. Open, is some gnarly fescue grass.
The fescue is knee-high or taller in some places, with Fox Sports' Ken Brown measuring portions of it in a pre-recorded segment airing during the U.S. Open telecast. By his measurement, the deep stuff just off the edge of the primary rough is 25.5 inches long. Basically, that fescue grass is 2 feet tall.
Those particular section of grass are typically that deep on the Wisconsin track, but the USGA has thickened it substantially just off the line of the primary rough, the 3- or 4-inch deep stuff you'd usually find at a U.S. Open. Subsequently, hitting that taller fescue grass with water over the nearly nine months the course had been closed to the public before the U.S. Open made it particularly thick and nearly impossible to hit out of successfully.
When the public goes to Erin Hills to play golf, they'll face this kind of grass -- which the course routinely cuts and dries and sells as hay -- but they won't face it when it's this thick.
Not only is it difficult to find the ball without a spotter or a caddie, it is tough for a golfer of any caliber to have enough clubhead speed to get through the fescue and even hit the ball. On top of that, it's nearly impossible to be able to hold the clubface open long enough to get it square at impact after the grass grabs the club and turns the face closed.