kavo

A dip, not a hole through. This is the distinction to fix first. Truo goes through something: a hole in a sock, a hole in the wall, daylight on the far side. A kavo is a hollow, a dip, a depression - the ground sinking away, a pothole in the road, the hollow of a tree, the socket of an eye. Nothing passes through it. English lets hole do both jobs and so an English speaker reaches for truo every time; half the time the word wanted is kavo.

The adjective is the older half of the pair. Kava means hollow, and it is at least as common as the noun: kava arbo, a hollow tree; kava vango, a sunken cheek; kava sono, a hollow sound. Kavigi is to hollow something out, elkavigi to gouge it out, kaveto a small dip - the dimple in a cheek, which is also kaveto. Kavaĵo is a hollowed thing.

And yes, kaverno is a relative - the seam is Latin, not Esperanto. Learners are told so often that similar-looking Esperanto words are unrelated that it is worth saying clearly when they are. Latin cavus, hollow, produced cavea, a hollow place, which produced caverna, a cave. Esperanto borrowed the short adjective as kav- and the long noun as kaverno, separately, centuries after they had parted company. So the two words genuinely share an ancestor, and you still cannot build one from the other: there is no -ern- suffix, and kaverno is one indivisible root. Honest answer: related by history, not by grammar. The same Latin adjective is inside English cave, cavity, concave, excavate and, by a longer road, cage.

Sizes, from small to large. A kaveto is a dimple or a dent. A kavo is a hollow you could put your hand or your foot in. A kaverno is a cave you can walk into. A karsta kaverno is the limestone kind. And abismo is the one with no bottom, which is Greek rather than Latin and belongs to a different family entirely.