THE RIDGE HOLES: 10-22mts.

The dive

The Ridge Holes is named for its one real feature. Waterworld's own description is of a series of very large round deep holes running along the top of a ridge, often full of crawfish or spiny spider crab, on rock formations the centre rates highly and among fish it says can be almost anything. The published band is 10 to 22 m, the shallowest top of any of the deeper Maharees pins, which gives the dive an unusual shape: a ridge crest you follow and look into, rather than a wall you swim along.

The rock is limestone, and the 1996 BioMar survey of the Magharee Islands describes it in the same terms at every station it dived here: bedrock thrown into ridges with intervening furrows, sharp-topped, and heavily pitted and bored. At the station nearest this end of the group the furrows bottomed at about 21.6 m and the ridge tops rose to 18.7 m, red algae dominated the rock, and there was a notable lack of brown algae. That is a good match for the geometry the centre describes, though the survey's pitting is at a much smaller scale than the holes the site is named for.

This is one of six names Waterworld publishes within about 750 m of each other in the middle of the island group, with The Point 200 m due south, Pollack Alley 180 m away and the Gully 310 m away. The centre gives each a distinct description and its own depth band, and works them as separate dives; they are best understood as named sections of one reef rather than six separate reefs. Nothing outside the centre's own listings names the Ridge Holes, so what is written here about this stretch specifically rests on the operator.

What the pin has on its own account is the animals. Grey seal is the single most recorded species within 1.5 km of it, bottlenose dolphin is close behind, and basking shark is on the record. Below that the list is the standard cold-water reef: edible crab, spiny spider crab, European lobster, velvet swimming crab, lesser-spotted dogfish, leopard-spotted goby, the sea cucumber Aslia lefevrei, common sunstar and the boring sponge Cliona celata. Access is by boat, ten to fifteen minutes out from Scraggane Pier at Fahamore.

• Very large round deep holes along the crest of the ridge
• Crawfish and spiny spider crab sitting in the holes
• Sharp-topped limestone ridges with furrows between them
• Shallow 10 m top, so the profile can be stretched out
• Grey seal, bottlenose dolphin and basking shark all on record at this pin

Conditions

Waterworld gives visibility across its Maharees sites as about 40 m on very good days, around 20 m on average and about 7 m on bad ones; the Castlegregory village guide reproduces the same figures but caps the best days at 30 m, so plan on 20 m. Water is about 8 °C at the start of the season and 17 to 18 °C in July, August and September. Boats leave Scraggane Pier at 9am and 11am with a ten to fifteen minute run. The islands face north into open Atlantic water: swell decides whether the boat goes, tide decides the hour.

On current: The sources disagree about how tidal this part of the group is, and the conservative reading wins. Dublin University Sub-Aqua Club, which dived the Maharees in June 2009, records the sites as fairly non-tidal with only a small drift even at mid tide. The older Dive Ireland guide reports a good current on the third and fourth hours of the tide in this middle section of the islands and advises diving only at slack water, and the 1996 BioMar survey records moderately strong tidal streams in Magharee Sound and describes the channels around the neighbouring islands as tide-swept. Take the boat's slack-water timing.

Best months: May, Jun, Jul, Aug, Sep