You cut into a sourdough loaf expecting an evenly aerated crumb, but discover one enormous tunnel directly beneath the upper crust. The rest of the crumb may be dense, slightly gummy, or filled with much smaller holes.

This defect is sometimes called a “flying crust.” It is not the same as the irregular open crumb many bakers intentionally try to achieve. A good open crumb contains holes of different sizes distributed throughout the loaf. A large empty space under the crust usually means that fermentation or shaping caused the gas to collect in one area instead of spreading evenly through the dough.

In most cases, the main cause is underproofing, although shaping technique, starter condition, and dough strength can also contribute.

Underproofing is the most common reason for a large tunnel beneath the crust.

If the dough enters the oven before fermentation has progressed far enough, it still contains a large supply of unfermented sugars and highly active microorganisms. Once exposed to the heat, gas production and expansion happen very quickly.

The outer surface begins to set while the inside of the loaf is still expanding. Pressure builds beneath the crust, and the gas follows the easiest path, creating one large empty pocket.

An underproofed loaf often has more than one visible sign. The bread may have strong oven spring but a dense lower crumb. The score may tear unpredictably, the sides or bottom may split, and the crumb can feel slightly tight or rubbery around the large hole.

Do not judge proofing only by the number of hours in a recipe. Dough temperature, starter strength, flour, and the amount of starter all affect fermentation speed.

Before baking, the dough should feel lighter and aerated rather than heavy and tight. During bulk fermentation, look for bubbles along the sides, a smoother and slightly domed surface, and a noticeable increase in volume. Depending on the recipe, an increase of approximately 30–50% may be enough, especially when a cold final proof is planned.

Sometimes fermentation is progressing well, but a large bubble remains near the surface during shaping.

Very gentle handling is often recommended for sourdough, but this does not mean that every bubble must be preserved. If one oversized gas pocket is folded into the dough and remains close to the outer surface, it can expand dramatically in the oven.

Before the final shaping, inspect the dough for unusually large bubbles. Gently press or pinch only those bubbles while preserving the smaller ones distributed through the dough.

The goal is not to flatten the entire loaf. It is to distribute the gas more evenly so that one bubble does not become a tunnel beneath the crust.

A hidden gap can also form when the dough is folded or rolled without enough contact between the layers.

If flour is scattered heavily over the dough during shaping, the surfaces may not stick together. The outside of the loaf can look smooth while a dry, flour-coated pocket remains inside. During baking, this gap expands and becomes a long hole under the crust.

Use only enough bench flour to prevent sticking. Try to keep excess flour away from the surfaces that will be folded inside the loaf.

As you shape, create steady tension without pulling the outer skin so tightly that it tears. Each fold should make contact with the dough beneath it. The finished loaf should feel supported and compact, but not compressed.

A dough needs enough strength to retain and distribute the gases produced during fermentation.

If the gluten structure is weak, gas bubbles can merge into larger pockets. This may happen when the flour is too weak for the amount of water, the dough has not developed enough strength, or fermentation has continued until excessive acidity begins to damage the gluten.

The dough may look loose, sticky, and difficult to shape. It may spread quickly after being turned out of the proofing basket.

Building strength does not always require intensive kneading. Proper mixing, several well-timed folds during the first part of bulk fermentation, and suitable hydration are often enough.

If the dough continually spreads and produces large tunnels, reduce the water slightly and observe whether the crumb becomes more even. Hydration should suit the flour rather than a number copied from another recipe.

A starter can rise in its jar and still ferment dough unevenly.

If it is repeatedly used long after its peak, stored hungry, or allowed to become excessively acidic, it may weaken the dough before the loaf develops enough volume. The result can be a combination of dense crumb and a few exaggerated holes.

Use a starter that rises predictably after feeding and has a clean, pleasantly fermented aroma. Ideally, add it to the dough near its peak, while the surface is still slightly rounded or has just begun to flatten.

If the starter smells extremely sour, alcoholic, or like acetone, give it two or three regular feedings before baking.

A hot oven is essential for good sourdough, but extreme heat can exaggerate an existing proofing problem.

If the crust sets very quickly while the centre is still cold and rapidly expanding, gas can become trapped beneath the upper surface. However, oven temperature alone is rarely the main cause of a flying crust.

First check fermentation and shaping. If those appear correct but the problem continues, verify the oven temperature with a separate thermometer. Household ovens often run hotter or cooler than their settings suggest.

Steam also delays crust formation and allows the loaf to expand more evenly. In a dry oven, the surface can harden too soon and restrict controlled expansion.

The next time you bake, allow the dough to ferment until it looks and feels aerated rather than relying only on the clock. Watch the dough temperature and give it additional time if your kitchen is cool.

During shaping, remove only very large bubbles, avoid trapping excess flour between the folds, and make sure the layers of dough remain in contact. Create enough surface tension to support the loaf without forcing all the gas into one area.

If you are unsure whether the dough needs more proofing, observe several signs together. Properly proofed dough feels lighter, shows visible aeration, and responds gradually when gently pressed. A very tight dough that springs back immediately usually needs more time.

One large hole beneath the crust is not a sign that you have finally achieved an open crumb. It is usually a message from the dough: fermentation needed more time, or the gas needed to be distributed more evenly during shaping. Once those two stages are corrected, the crumb usually becomes much more balanced.