Earlier in the week, speculation was that rain and wet course conditions would make for a lot of low scores at Merion, but players have found that there's more to this old course than the speed of the greens.
The average score for the first round was 74.14, or slightly more than four strokes above par. [Ireccomend checking out this piece from Brian McCrone on the tough conditions, and this one from Justin Klugh on the players' respect for and fear of Merion.]
Furthermore, there are a pair of par fours - the 504-yard fifth and the 521-yard 18th - that have been tougher than the rest, and could be among the toughest in recent history. And there are two others that are right up there.
The USGA said they only began keeping those statistics in the 80's, and don't have them available to pull the toughest holes. I was, however, able to find some numbers from recent Opens, and here's how they stack up:
+.78 - 5th at Merion East, 2013 (Par 4, 504 yards)
+.77 - 18th at Merion East, 2013 (Par 4, 521 yards)
+.60 - 18th at Oakmont, 2007 (Par 4, 484 yards)
+.55 - 5th at Pinehurst No. 2, 1999 (Par 4, 476 yards)
+.49 - 17th at Pebble Beach, 2010 (Par 3, 208 yards)
+.47 - 6th at Merion East, 2013 (Par 4, 487 yards)
+.47 - 17th at Merion East, 2013 (Par 3, 246 yards)
+.40 - 18th at Congressional, 2011 (Par 4, 523 yards)
So it clearly hasn't been easy out there, but it could be worse. The USGA told us that during the 1950 U.S. Open, the par-five fourth hole (628 yards) played to more than a stroke over par, with an average score of 6.16. This year, probably because players hit the ball a lot longer than they used to, is playing to a much more reasonable .31 strokes over par.
The U.S. Open record for most total rounds over par is also in jeopardy, as all but six players were over par after the first round, and there could be even fewer today. That record (446) was set at Oakland Hills in 1996.