Coral Grotto, also called Turtle Grotto, is described by Heron's guides as a playground for all divers. It is a scenic coral wall cut by gutters, and the classic route zig-zags in and out of the gutters, so the dive constantly changes between open wall and enclosed corridors of coral. Depths run from about 8 to 18 m.

The site's alternative name tells you what to expect: turtles are everywhere, asleep under ledges, swimming along the wall or feeding on the reef. Turtles visit daily to rest and feed, and it is not unusual to see several on a single dive. The ledges also shelter wobbegong sharks and the less common epaulette shark, while whitetip reef sharks patrol regularly along the wall.

Manta rays occasionally pass through in the blue beyond the reef edge. Reef fish are abundant in the gutters, and the wall is well covered in hard and soft corals. Coral Grotto suits divers of every level, from recently certified divers to photographers, because it combines easy depth with an interesting layout and near-guaranteed turtle encounters.

The gutters are open to the surface and do not involve any overhead environment. It is one of the less-publicised sites on Heron Reef but one that many repeat visitors ask for, particularly during the turtle nesting season (November to March), when the numbers of adult turtles around the reef peak. Gutters like these also concentrate reef fish and small invertebrates, so it is worth pausing at their inner ends to look under the coral ledges before moving on to the next gutter.

Heron Island is 72 km north-east of Gladstone. Fly or drive to Gladstone, then take the Heron Islander ferry from the Gladstone Marina terminal (72 Bryan Jordan Drive; check-in from 7:45 am, departs 9:30 am on scheduled days, about 2 hours), or transfer by seaplane or helicopter. Dive gear is carried on the ferry in addition to baggage.

On the island, the dive centre sits beside the harbour; boats leave the jetty for three dives daily and most Heron Reef sites are 5-15 minutes away. Coral Grotto is a short boat ride (5-10 minutes) from the jetty and is dived from a mooring.

Heron Reef sites are chosen on the day by wind and tide. The prevailing south-east trade winds (strongest April-September, often 15-25 knots) make the southern and south-eastern reef edge rough, so the sheltered northern and western sites are dived most. Northerly winds in spring and summer reverse this.

The tidal range is up to 3-4 m and currents run strongly around the reef edge and through Wistari Channel on springs; visibility is usually best on the incoming tide. Cyclone season is November-April; summer storms and plankton blooms can drop visibility to 10 m. Stingers are uncommon this far south but present in summer.

Descend to the base of the wall and zig-zag through the gutters, alternating between the wall face and the gutter floors, checking under ledges for wobbegong and epaulette sharks. Watch the blue for passing mantas, then return along the top of the wall.

Heron Island Resort Dive Centre is the only operator: three guided boat dives a day (about 9 am, 11 am and 3 pm) plus scheduled night dives, from the resort jetty; sites are rotated by tide and wind. Access requires a stay at Heron Island Resort (or the University of Queensland research station).