Drywood termites can cause heavy damage to wood if left undisturbed for a long time. The damage is correlated with their colony size. As the colony gets larger, the damage increases, even though it generally takes several years before the colony reaches a size large enough to cause noticeable damage.
The termites hollow out the wood in the process of constructing chambers and galleries, although the damage will be localized in all cases. The appearance of massive damage all over a place within a building, is almost always caused by multiple colonies of drywood termites, and not due to one single large colony carrying out all the damage.
These pictures are of damage by drywood termites from the Cryptotermes genus, although generally, damage caused by any species of drywood termite looks similar, and develops in a similar manner.