Secca del Mariuolo

The dive

The Mariuolo is a modest rock breaking the surface a very short way off the coast of Zannone, near Capo Negro, and it is the visible part of a mass that widens underwater and breaks into large boulders. The marine biologist Adriano Madonna, writing about it in 2014, admits he does not know where the name comes from. Zannone is the northernmost of the Pontine islands and has belonged to the Parco Nazionale del Circeo since 1979; it is a boat run north from Ponza, and the dive is one that operators going that way put on the programme rather than a standard Ponza site.

The structure is what makes the dive. Ponza Diving describes ledges cut with small caves and fissures, a cap at 4 m, a vertical section to about 19 m, and a ridge that runs from 25 m down past 42. Il Bucaniere gives the site a medium-high grade and a maximum of 45 m, and names the two features everyone remembers: a natural arch, and a cave you can swim through at 30 m. Madonna records a cave entrance a comfortable 2 by 3 metres and a bottom varying between 25 and 35 m. The published maxima do not agree, which is normal for a site with a shallow top and a ridge that keeps going; treat 40 m and below as the deep end and decide before you get in.

What you see follows the depth. Ponza Diving reports scorpionfish inside the through-cave and a nearby pinnacle where dozens of seabream gather; Madonna adds crustaceans and flatfish sitting camouflaged on the sand between the boulders. Il Bucaniere lists lobster, gorgonians, dentex, barracuda, grouper and amberjack. Our occurrence data is no use here: two records inside the search radius, which tells us the area is undersampled rather than empty, so nothing in this page rests on it.

• Through-cave at about 30 m, with scorpionfish inside it
• A natural arch in the rock
• A cap at 4 m, so the shallow end of the dive is a real dive rather than an ascent
• A pinnacle beside the main rock where seabream gather in numbers

Conditions

A boat run north from Ponza to Zannone, then a dive on an isolated rock close to the island's coast. The profile is naturally multi-level: down the ridge or through the cave first, then back up onto the cap at 4 m to finish. Mediterranean scrub, broom, prickly pear and agave on the slopes above are what you look at on the surface interval, because there is nothing else there.

On current: No source we read gives a current rating for this rock, so we give none rather than guess. The published depths do disagree and that is worth knowing before the briefing: Ponza Diving puts the cap at 4 m, a vertical drop to about 19 m and a ridge running from 25 m to beyond 42; Il Bucaniere gives a flat maximum of 45 m; Madonna, writing in 2014, describes a bottom between 25 and 35 m. We carry 45 m as the maximum because on a site like this the safe assumption is the deeper one.