Troll-holes

For those kinds of Trolls affected by the Sun, such as Stone-trolls, a dark refuge was needed in which they could escape being turned to stone during the day. Such a hole was used by Bert, Bill and Tom, the three Trolls encountered by Bilbo in the woods of Trollshaws. Also used by the Trolls as their larder and their treasure hoard, the Troll-hole was filthy and bone-strewn, but it was more than a simple cavern: the Trolls had at least gone to the trouble of fitting it with a solid, lockable door.

This is the only Troll-hole of which we have any distinct description, but it seems that other Trolls tended to make broadly similar refuges (when Pippin came upon the same Troll-hole decades later, he immediately recognised it for what it was, so Troll-holes must have followed a fairly similar pattern from Troll to Troll.)