You cut into a sourdough loaf expecting an open, even crumb. Instead, you find several enormous holes beneath the crust while the rest of the loaf is tight, heavy, and almost gummy.

At first, those large holes may look impressive. Open crumb is often treated as a sign of successful sourdough. But a few tunnels surrounded by dense dough do not mean the loaf fermented well. They usually show that gas developed unevenly or became trapped in places where it could not spread through the gluten network.

A healthy open crumb contains bubbles of different sizes distributed throughout the loaf. When the difference between the largest holes and the densest areas is extreme, something happened during fermentation, folding, or shaping.

Uneven crumb is often connected with insufficient bulk fermentation.

During bulk fermentation, gas must develop throughout the dough. At the same time, the gluten network becomes more elastic and capable of holding that gas. If the dough is shaped too early, some areas remain dense while a few active pockets continue expanding.

The loaf may appear to rise during the final proof, but the internal structure is still uneven. In the oven, the largest bubbles expand dramatically and create tunnels, while the underfermented areas remain compact.

Before shaping, look for more than a few bubbles on the surface. The dough should feel lighter, slightly aerated, and elastic throughout. Its edges should be rounded rather than flat, and bubbles should be visible along the sides of the container.

The dough does not always need to double, but it should show a clear and even change from the moment bulk fermentation began.

A large tunnel directly under the top crust often forms when gas collects near the surface of the dough.

This can happen when the outer layer becomes strong while the dough beneath it remains underfermented. The expanding gas moves upward and becomes trapped under the skin instead of being held in smaller pockets throughout the loaf.

High dough temperature can make the problem more noticeable. The outer and warmer areas may ferment faster than the cooler centre, especially when the dough was not mixed thoroughly or the fermentation container is standing near a heat source.

Try to keep the dough at an even temperature. When using warm water, mix carefully so that no warmer or cooler areas remain. A straight-sided container can also help you observe whether fermentation is progressing evenly.

Stretch-and-folds and coil folds strengthen the dough, but they also redistribute temperature, yeast activity, and gas.

If the dough receives no folds, weak sections may remain unable to hold small bubbles. Gas then travels toward stronger areas and forms larger pockets. If the folds are too aggressive or continue late into bulk fermentation, they can push gas into isolated areas or damage the structure that has already formed.

Most doughs benefit from several gentle folds during the earlier part of bulk fermentation. Each fold should make the dough smoother and more cohesive. Once it holds its shape and fermentation becomes visibly active, it usually needs rest more than additional handling.

The goal is not to remove every bubble. It is to create a continuous gluten network that can support bubbles throughout the dough.

Sometimes the problem develops during shaping rather than fermentation.

When flour is trapped between folded layers of dough, those layers may not stick together. A pocket remains between them and expands into a long tunnel during baking. Large visible bubbles on the surface can also become trapped inside if they are folded into the centre of the loaf.

You do not need to press all the gas out of well-fermented dough. However, unusually large surface bubbles can be gently released before final shaping. Use enough tension to bring the layers together while preserving the smaller bubbles inside.

Too much flour on the work surface makes this more difficult. Flour should prevent serious sticking, but the inner surfaces of the dough need enough contact to seal.

Wet dough naturally produces a more open and irregular crumb, but additional water cannot create good structure by itself.

If the flour is not strong enough for the hydration, the gluten network may be unable to distribute gas evenly. Some bubbles merge into large cavities while other sections collapse and become dense.

This is why increasing hydration does not always produce a better open crumb. It may simply make the dough harder to ferment and shape consistently.

If the loaf repeatedly contains tunnels and dense areas, reduce the water slightly. A dough that is easier to strengthen and shape often produces a more attractive crumb than a wetter dough that cannot support itself.

The location of the holes provides useful clues.

A long tunnel directly beneath the crust often suggests underfermentation or trapped gas during shaping. Large holes near the top with a dense base usually indicate that the dough needed more bulk fermentation.

A few isolated cavities between otherwise well-fermented layers may come from excess flour during shaping. If the entire crumb is very open but weak and slightly gummy, the dough may have fermented too long or lacked enough gluten strength.

Look at the crumb together with the loaf’s height, oven spring, and the way the dough behaved before baking. One feature alone rarely provides the complete diagnosis.

Large holes are not automatically a sign of excellent sourdough, just as a fine crumb is not automatically a failure. What matters is whether the structure suits the bread and whether fermentation has developed evenly.

Give the dough enough time during bulk fermentation, strengthen it gently, keep its temperature consistent, and avoid trapping flour or oversized bubbles during shaping.

When gas is distributed through a continuous gluten network, the crumb becomes lighter and more balanced. It can still contain beautiful large pores—but they will belong to the structure of the loaf rather than appearing as empty tunnels above dense, underfermented dough.