Were you looking for the mystery web, a rare drop of Mirkwood Spiders?
|“||He had picked his way stealthily for some distance, when he noticed a place of dense black shadow ahead of him, black even for that forest, like a patch of midnight that had never been cleared away. As he drew nearer, he saw that it was made by spider-webs one behind and over and tangled with another.||”|
–The Hobbit, Flies and Spiders
Like normal cobweb blocks, these blocks slow down the player and other mobs; but unlike cobwebs, neither shears nor most swords can break the Web of Ungoliant any faster than you can break it by hand. Since Update 34, shears can harvest it.
A little-known fact is that the sword Sting is effective at destroying these webs – however, since this sword is currently unobtainable, this fact is of little use.
The most effective way of getting out is by placing a bucket of water next to the web which destroys the web instantly.