A cave under the crust means the shaping trapped a bubble

The theory of hollow gaps under the crust is well covered elsewhere. This is about the version you meet in practice.

What the void actually is

During bulk fermentation, gas collects unevenly, and some cells grow far larger than their neighbours. If one of those large pockets survives shaping intact, it sits just under the surface of the loaf.

In the oven that pocket expands along with everything else, but it has no small cells around it to distribute the pressure. The crust sets above it and the crumb sets below, leaving a long hollow following the shape of the loaf. It is a structural fault rather than a fermentation one, which is why it appears in otherwise well-made bread.

Why shaping is the usual cause

Preshaping and shaping are supposed to redistribute gas, breaking up large pockets into many smaller ones. Handling that is too gentle leaves the large cells intact, and handling that is too rough tears the network instead.

In the tin, rolling a loaf tightly over a slack, gassy interior traps whatever was there rather than redistributing it. The seam is a common site, since a poorly sealed seam lets gas collect along the join. Loaves rolled from a rectangle are more prone to it than those shaped from a round, because of the rolled layers.

Overproving and its contribution

An overproved dough has weakened cell walls that merge readily into much larger voids as the gas moves between them. It also has less structure to resist the expansion of a large pocket during oven spring.

That combination makes a cave considerably more likely even when the shaping itself was perfectly reasonable. A loaf that has both a cave and a flat profile is telling you the proof went too far. Shortening the final proof therefore addresses both the cave and the flat profile at the same time.

The tin loaf version

In a tin, the same fault shows as a gap along the top, sometimes lifting the whole crust away. Tin loaves are more prone to it because the dough is often rolled and dropped in without much redistribution. Pressing the dough out flat and rolling it firmly, sealing as you go, breaks up the pockets that would otherwise survive.

In the tin, some bakers dock the shaped dough with a skewer to release any large bubbles before proving.

That is a blunt fix and it works, though it costs a little of the rise you had built.

Getting the degassing right

The aim is to redistribute gas rather than to remove it, since the small cells are what you want to keep. Pressing gently and evenly across the whole surface breaks large pockets without flattening the dough. A preshape followed by a rest and then a final shape gives two opportunities to catch a large bubble.

In the tin, watching for visible bulges during the final proof lets you catch one before it goes into the oven. A pin or skewer through an obvious bubble late in the proof is a legitimate last-minute correction.

Distinguishing it from other faults

A cave under the top crust with an otherwise even crumb points squarely at shaping. Large irregular holes distributed through the whole crumb point at overfermentation or very gentle handling instead. A dense band at the base with an open top usually means overproving rather than a trapped bubble.

In the tin, a separation following the line of a rolled seam is a sealing fault at that specific point. The location of the void is the most informative thing about it, so look before you cut a second slice.

The takeaway

Redistribute the gas instead of trapping it, and look for bulges before the loaf reaches the oven.

Temperature and time are the same argument held at opposite ends.