"That was me. I used illusion to make myself so large and to change my appearance. The laces of my provision bags were tied with unbreakable iron wire and could be undone only by magic. When you hit me with your hammer, Thor, while I pretended to sleep, I knew that even the lightest of your blows would have meant my death, so I used my magic to take a mountain and put it invisibly between the hammer and my head. Look over there."
Far away was a mountain in the shape of a saddle, with valley's pluging into it: three square-shaped valleys, the blast one going deepest of all.
"That was the mountain I used," said Utgardaloki.

Thor’s Journey to the Land of the Giants