 This is an iceberg order sitting on the offer at 39.60. 170 contracts have already been filled by this hidden actor. As market buyers gain momentum and price approaches the iceberg, note how the book only displays around 100 sell orders at this level. A surge of aggressive buyers lift the offer with over 300 contracts yet the price level cannot be broken. The hidden iceberg order just absorbed all the buyers now with a fill of 497 contracts. Buyers have another go at breaking this level and they get completely absorbed once again. Here come the buyers again with around 700 more contracts yet the iceberg provides a solid defense. Buyers finally break through fully filling the iceberg order.