Alden's Cave is the best-known feature of the eastern side of Flinders Reef, and a good introduction to overhead environments for newer divers because it is short, bright and has an opening in its roof that lets daylight flood in. The mooring block sits just south of the entrance, so it is easy to find. Inside, the walls and ceiling are coated in encrusting sponges, soft corals and small invertebrates, and shafts of light through the roof hole make it a favourite with photographers when the water is clear.

Operators describe it as ideal for beginners, and it is regularly used as the second dive on a Flinders day when conditions allow a move from the sheltered western coral gardens to the more rugged eastern face. The eastern side of the reef has a different character to the west. Rather than open coral gardens, it is a landscape of rocky ledges, walls, small caves and gutters dropping into the mid-twenties, with more exposure to the open ocean and a better chance of larger animals.

Wobbegongs rest under the ledges, schools of sweetlip, trevally and batfish hang in the gutters, and eagle rays, leopard sharks (in summer) and occasional whaler sharks and grey nurse sharks pass along the edge. Turtles are common here as well as on the western side. Look for crayfish, nudibranchs and moray eels in the cracks.

Because the cave is short and has a clear exit, it does not need cave training, but divers should still keep good buoyancy to avoid damaging the growth and stirring up sediment. It combines well with the Cementco wreck, about 400 m to the north-east, on a two-dive day.

Boat access only. Brisbane charters leave from Manly Boat Harbour (Brisbane Scuba departs from the corner of Royal Esplanade and Davenport Drive, Manly) or from Scarborough Boat Harbour on the Redcliffe Peninsula, crossing Moreton Bay and running out through the North West Channel past Bulwer and Comboyuro Point to the northern tip of Moreton Island. Allow roughly 75-90 minutes from Manly in good conditions.

Sunshine Coast operators run south from The Wharf, Mooloolaba, in about 45 minutes. Private boats typically launch at Scarborough or Bribie Island; the open-water crossing around Cape Moreton is exposed and should only be attempted in settled weather with local knowledge. Alden's Cave is reached from the mooring on the eastern side of the reef, with the mooring block a short swim south of the cave entrance.

The eastern side of Flinders faces the prevailing south-east to easterly swell, so Alden's Cave is only dived on calmer days – as a rule swell under about 1.5 m and winds under 15 knots, ideally from the west. North-easterly winds and easterly groundswell make the eastern mooring rough and create surge in the cave. A light current can run along the eastern face.

Winter westerly mornings give the best conditions and visibility of 20-30 m; summer averages around 15 m.

Descend the eastern mooring, locate the block and swim north a short distance to the cave entrance. Pass through slowly, looking up at the opening in the roof, then continue along the eastern ledges and gutters before turning back to the mooring at half gas. Safety stop on the line.

Brisbane-based operators (Brisbane Scuba, Go Dive Brisbane, Brisbane Dive Academy) and Mooloolaba operators visit on Flinders Reef days when the eastern side is calm.