One indicator of the high quality and functionality of a knife is the number of special operations units around the world that use them.

An Italian company called Extrema Ratio offers a very well-designed knife for special operations, even to include special ops helicopter pilots. One type of special ops unit that has traditionally made substantial use of Extrema knives is the combat swimmer/combat diver unit. For them, Extrema Ratio has developed the Ultramarine. Working in conjunction with Italian military and law enforcement divers, the Ultramarine was developed to replace multiple knives or tools likely to be needed by the combat diver.

The resulting knife is designed to fulfill the five basic functions identified as important for combat divers—general utility/field usage, underwater operations, emergency and rescue, survival, close combat and parachute/helicopter insertions. Note that these functions are listed in the order Extrema Ratio designers determined the blade would be used most frequently. For ease of explaining the features, I’ll discuss each function separately.

Extrema Ratio’s Ultramarine in its element—the water.


So that the Ultramarine can be used as a working tool, the blade was made heavy enough to chop branches or shrubs for use in building shelters or camouflage. Blade serrations make cutting rope easier. The steel pommel may also be used for hammering or breaking glass. The sharp portion of the blade near the handle may be used for precision cutting tasks or for shaving twigs to get tinder, etc.


The sheath was designed with a flap to prevent sharp edges from damaging rubber boats or buoyancy devices. The slot in the blade was specifically incorporated for use as a screw-pin shackle wrench. There is also a retention lanyard, which allows the user to keep control of the knife without having to sheath it when carrying out tasks requiring both hands.

One side of the handle is also equipped with a retention strap through which the hand may be readily slipped. The Ultramarine’s oversized grip and anti-slip grip material help retain control of the knife while wearing neoprene diver’s gloves. The blade was also tested for prying strength. Toward the tip, where the line cutter is incorporated, the Ultramarine will still withstand a 99-pound load. Toward the center, where the shackle wrench is located, it will withstand a 154-pound load.

Ultramarine with its sheath. Flap covers knife so its edges do not puncture rubber boats. Red tab allows retention ring to be quickly released in an emergency.


The Ultramarine is designed with a net or line cutting system should the diver become entangled. To allow rapid access and deployment of the knife, even though the sheath has various retention systems, in an emergency a quick-release pull tab may be employed. Both the sheath and the lack of a protruding handguard make the Ultramarine much less likely to snare. The handle is also designed so that, even in black water, the operator can distinguish the orientation of the knife by feel. The pointed pommel of the Ultramarine also functions as an effective glass breaker.

Ultramarine’s features include blade with slot for screw-pin shackles, serrations for rope cutting, line-cutting slot, non-slip handle, and pommel/tang, which may be used for glass breaking or as an impact weapon.


For survival fishing, the blade may be attached to a pole to create a spear. For use in a combat environment and for escape and evasion, the matte black finish of the Ultramarine makes it very non-reflective. Sharpeners for both the primary blade and net/line cutters are incorporated into the sheath.


The spear-point, double-edged blade allows the Ultramarine to be used effectively for slashes or thrusts. The handle-heavy balance of the knife is also an aid in most forms of knife combat. Additionally, the pointed steel pommel can function as a “skull crusher” impact weapon.


The combination of the hard sheath and its streamlined design makes it less likely to become entangled with equipment or rigging during a parachute or helicopter fast rope insertion.


The Ultramarine is tough and sturdy but also compact. Overall length is 11.75 inches, with the blade accounting for 5.9 inches of that. Blade thickness is .25 inches. Overall length of the sheath with the knife inside is 12.6 inches.

Extrema knives are well known for the quality of their steel. The Ultramarine is fabricated of Bohler N690. Note there is also an EOD version of the knife fabricated from Titanium. The handle is of Polyamide nylon. The hard sheath is of ABS and the soft sheath a combo of Cordura, polyamide, and polyethylene foam.

Spain’s combat swimmer unit UOE uses Ultramarine.

The Ultramarine package includes various accessories and spares: two TEK-LOK belt supports, two additional leg straps, one Lubra-Skin spray, one ink eraser, two additional O-rings, one additional TEK-LOK screw, one additional washer, one additional female threading and one additional red Velcro strap.

I would be lying if I said I was unbiased in evaluating Extrema Ratio knives. I have been using a couple of Extrema knives for years, and they have made me a great fan of the brand. But I’m certainly not alone in my confidence in these knives. Among the special units that use the Ultramarine are the divers of the Australian Special Air Service Regiment and Spain’s combat swimmers, the UOE.

Another thing I like about Extrema Ratio knives is that not only are they tough and very functional, but they also have a bit of Italian design flair. Based on my own experience, the experiences of friends in units that issue Extrema Ratio knives, and the experiences of friends who have bought these knives and used them in combat zones, I recommend them highly.

Extrema Ratio in Italy is listed as the source because their U.S. distributor has changed frequently. My recommendation is that anyone interested in obtaining an Ultramarine or any other Extrema knife should order it direct from Italy. Extrema’s website is in English and the staff who deal with foreign orders speak English and can answer questions. Best of all, shipping is very fast. I have ordered a knife from Extrema Ratio in Italy on the same day I ordered another product from a company in the U.S. The package from Extrema arrived first.


Extrema Ratio
Dept. S.W.A.T.
Via Tourcoing, 40/P
59100 Prato (PO) ITALY
011-39 -0574-584639

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