Siaes Tunnel

A 90-metre natural tunnel cutting through the western barrier reef, entered at 35 meters and exiting onto an open wall patrolled by grey reef sharks

Overview

Siaes Tunnel is a natural passage carved through the reef on Palau’s western barrier reef, northwest of Ulong Island, approximately 16 miles (26 km) from Koror. The tunnel entrance sits at around 100 feet (35 meters) depth on the reef wall, and the passage runs approximately 90 meters through the reef structure before opening onto the outer wall. Two large natural windows cut into the tunnel sides allow ambient light to filter in throughout the transit, preventing true blackout conditions. The exit onto the outer wall delivers a classic Palau moment: emerging into brilliant blue water with the wall dropping beyond recreational limits below and grey reef sharks and pyramid butterflyfish visible in the blue. Siaes Tunnel is classified as an advanced dive due to its depth — the tunnel floor reaches approximately 40 meters — and is typically dived in combination with the nearby Siaes Corner.

Site Information

• Location: Northwest of Ulong Island, western barrier reef, Palau, Micronesia
• Entry Type: Boat dive
• Site Type: Cave/cavern
• Difficulty Level: Advanced
• Maximum Depth: 40 meters
• Typical Visibility: 15–30 meters (50–100 feet)
• Current: Light inside tunnel; moderate to strong on outer wall
• Best Time: Year-round; best light effects with overhead sun

Marine Life

Inside the tunnel, whitetip reef sharks and stingrays are found resting on the sandy tunnel floor. Big-eyed jacks and snappers school at the entrance. Rare deep-water pygmy angelfish (including Centropyge multicolor) have been recorded on the tunnel walls and surrounding reef. Pyramid butterflyfish and anthias populate the wall sections near the tunnel exit. Watchman gobies with alpheid shrimp are found in crevices. Grey reef sharks patrol the outer wall beyond the exit. Sea fans and soft corals decorate the tunnel walls where light penetrates.

Dive Profile

Descend the wall to the tunnel entrance at approximately 35 meters. Swim single-file through the 90-meter passage, maintaining careful buoyancy to avoid contact with the ceiling and walls. The tunnel floor reaches a maximum of approximately 40 meters at its deepest point. Two natural windows provide natural light throughout. The exit opens onto the outer reef wall. After emerging, divers can drift along the wall or proceed to Siaes Corner if air and dive profile permit. Given the depth, bottom time inside the tunnel is limited — plan conservatively. Safety stop at 5 meters on the reef top on return.

Entry and Exit

Boat dive from Koror (approximately 35–45 minutes) or from liveaboards. Entry by giant stride with descent along the wall to the tunnel entrance. Exit is typically a drift pickup after the wall portion, with SMB deployment at 5 meters. Advance deep diving specialty certification is strongly recommended.

Tips and Recommendations

• Carry a primary dive torch and at least one backup — sections of the tunnel away from the windows are dim
• Stay single-file through the tunnel and maintain precise buoyancy to avoid silt and formation contact
• Pause at the tunnel exit before emerging — sharks are often visible in the blue beyond
• Combine this dive with Siaes Corner if air and profile permit
• Nitrox is strongly recommended to manage the significant depth and extend usable bottom time

Safety Considerations

This is a genuine overhead-environment dive at serious recreational depth (35–40m). The combination of depth, overhead environment, and current demands Advanced Open Water certification at minimum; a deep diving specialty and experience in overhead environments are strongly recommended. Narcosis can affect judgment at 35–40m — know your limits. Carry redundant lights. Maintain the rule of thirds for gas management. The outer wall can have strong currents — be prepared for the transition from the sheltered tunnel to exposed wall. Do not attempt this dive without a local guide familiar with the tunnel layout. Advanced certification and current-diving experience are required.

Photography

The tunnel offers dramatic through-the-looking-glass compositions. Shoot toward the exit opening for a framed view of the blue water and wall beyond, with a diver silhouetted in the opening. Wide-angle or fisheye is essential inside the tunnel. The two natural windows provide rim-lighting opportunities. Outside on the wall, wide-angle captures sharks and pyramid butterflyfish against the blue. The contrast between the dim tunnel interior and the bright exit creates compelling images.

Sources: Fish N Fins Palau, Palau Dive Adventures, Just Gotta Dive. Last updated 2026-06-25.