Tunnels Beach reef is divided into two main zones: an inner reef and an outer reef. These zones are close to each other but behave very differently. Knowing the difference is important for safe snorkeling.
First-time snorkelers do best by staying shallow, using sand channels, and treating the outer reef as a boundary rather than a destination.
FRINGING REEF
The reef at Tunnels Beach is a fringing reef. It grows directly on top of old lava that flowed into the ocean from the shoreline long ago.
As the lava cooled, it cracked, collapsed, and formed tubes. Coral later settled on this uneven rock.
Because of this, the reef is not smooth or even. It is made up of coral patches, lava ridges, holes, and sand channels. These features are part of the natural reef structure, not signs of damage.
Coral grows in separate clusters with sand in between. The sand moves over time, changing the shape of the reef floor. During low tide, some coral heads come very close to the surface. This is why standing on the reef causes harm and why snorkelers are taught to float instead of walk.
TWO REEF ZONES
Tunnels Beach reef is divided into two main zones: an inner reef and an outer reef.
These zones are close to each other but behave very differently. Knowing the difference is important for safe snorkeling.
Understanding the reef system helps snorkelers stay safe while protecting the coral.
INNER REEF
The inner reef begins just past the sandy beach. This is the shallow area where most snorkeling takes place. Water depth here is often only a few feet.
Sandy channels are used for entry because they reduce contact with coral. Staying close to shore keeps snorkelers inside the inner reef, where conditions are more stable. The reef structure itself sets natural limits on where it is safe to snorkel.
OUTER REEEF
Farther out is the outer reef, where the lava base is more solid and the water is deeper. This area is more exposed to waves and water movement. In winter, large waves from the North Pacific hit this outer reef directly. These waves break coral, move sand, and reshape the reef. This process is normal and happens every year.
THE TUNNELS
The name Tunnels Beach comes from a series of underwater lava tubes and arches formed long before coral grew in this area. These tunnels were created when lava flowed into the ocean, cooled on the outside, and continued moving underneath. When the lava drained away, hollow passages were left behind. Over time, parts of these tubes collapsed, creating openings, arches, and short swim-throughs.
Coral later grew on top of this lava structure. Instead of filling the tunnels, coral settled around them, leaving the hollow spaces intact. This is why the reef here feels three-dimensional rather than flat. The tunnels are not carved by water or people; they are part of the original volcanic foundation of the reef.
Most of the visible tunnels are located near the transition between the inner and outer reef. Some are shallow and can be seen from above, while others sit deeper and are only visible to confident swimmers in clear conditions. The openings can look calm, but water often moves through them due to wave surge, even on days that appear calm from shore.
For first-time snorkelers, the tunnels are best viewed from a distance. Entering or swimming through them requires strong swimming skills, good awareness of water movement, and proper depth control. The tunnels are narrow, uneven, and surrounded by coral that is easy to damage by contact. ocean conditions and land use in Hāʻena.
SEASONALITY
The reef at Tunnels Beach changes with the seasons. In summer, waves are smaller, sand settles, and visibility improves. Coral growth continues during this time. In winter, strong waves remove loose coral and sand, resetting the reef surface. Because of this cycle, the reef never stays exactly the same.
Since the reef is close to land, rain also affects it. Heavy rainfall sends sediment from the mountains into the ocean. This sediment can cover coral and reduce sunlight, which coral needs to survive. For this reason, reef health at Tunnels Beach depends on both ocean conditions and land use in Hāʻena.