As well as NUMA's fictional budget. Cussler runs the real-life NUMA, and would kill for the kind of cash that Pitt gets.
The fictional NUMA is a US Government department. Cussler's real life NUMA is a private organization with no official recognition.
And the startling number of blue and green eyed people in his works.
Author Avatar: Ever since Dragon, Cussler is a recurring character in the Pitt series, usually showing up to put the heroes back on track with a minor Deus ex Machina. May also count as a Self-Insert Fic, although Word of God says it grew out of an attempt to see just how much his editor would put up with.
Big Damn Heroes: Used in the very beginning of books. It usually goes like this: The Girl Of The Novel and her team has just landed in big trouble. They're in a life-or-death situatipn and no one can help them. Then Pitt and Giordino come along and save them, becoming wrapped up in their problem and kick-starting the plot.
Canon Discontinuity: A number of the early Dirk Pitt books end with sweeping scientific or political changes (the development of a Star Wars system that makes nuclear war impossible, the merging of the US and Canada into a single country) that are then completely ignored by subsequent books since they conflict with the by-and-large real-world setting.
In Cyclops, Dirk Pitt is hinted to have been born in 1951 (his given age was 38 in-universe 1989), yet Admiral Sandecker never ages, is always described as a man in the late middle age. In other books, the good ole' Admiral is described as born in 1918, which would make people wonder if he is human at all.
Also, the Titanic is raised whole in the book of the same name. After it was discovered in real life to have broken in half, and that raising it was impossible, the whole incident is erased from canon.
Casual Danger Dialogue: Pitt and Giordino are masters of this, especially when all the other characters are terrified out of their wits.
The Casanova: Subversion - Pitt always seems to patiently wait and let the woman do the first step, if ever.
Cool Boat: The Oregon definitely qualifies for this one. The series features lots of these, given that it centers around a marine science agency.
Girl Of The Novel (although Pitt has now settled down with Representative Loren Smith, the only girl he sleeps with in more than one story - some of his other lovers appear in multiple novels, but only sleep with him in one of them)
Of special note is Summer Moran, who was presumed dead at the end of Pacific Vortex, later turned out not only to have survived (though she later died offscreen), but been pregnant with two kids by Pitt, the grown-up versions of whom would later feature in the novel. Despite the fact that over the course of the book, they spent about two hours in the same room as each other, which weren't used doing the nasty.
Green Eyes: Dirk Pitt himself. They are described as "opaline" and "like the sea."
Hilarious in Hindsight: The new genetically enhanced salmon trying to get on the market are being called Frankenfish. Sounds like someone is a fan...
History Marches On: 1976's Raise The Titanic, and the movie based upon it, both assume the R.M.S. Titanic sank as a single piece, remaining intact enough to be salvaged. In Cussler's defense, this was the official position on the sinking until the ship was actually found, which was in 1985, almost 10 years after the book was written.
Karmic Death: Most of the villains get taken out in this manner.
Kiss Me, I'm Virtual: Hiram Yeager's computer GUI "Max" is a sometimes scantily-clad representation of his wife.
Lawyer-Friendly Cameo: In Night Probe, it is strongly implied that Brian Shaw (British agent) is really James Bond.
Leaning on the Fourth Wall: In some of the later books people looking over reports of Dirk's career mention that his life reads like a series of adventure novels.
Mineral MacGuffin: Several of the books, including Raise the Titanic and Arctic Drift, hinge on the need to find a supply of some exotic substance which seems to be available only in the sunken ship du jour.
One Steve Limit: Averted, there seems to be an infinite number of Leigh Hunts mentioned in the series (Though only one in any given book). Also, Dirk Pitt Jr. After he is introduced, the elder Dirk Pitt is referred to as Pitt and the younger as Dirk so that the reader can tell them apart in scenes where both are present.
Running Gag: Sandecker never finds any of his special cigars missing, but Giordino always seems to be smoking one. Al secretly tracked down the source of Sandecker's cigars and has been quietly purchasing them himself.
Series Continuity Error: While not exactly an error, Fridge Logic sets in when in one book there is a secret moonbase but in another the moonlandings were faked. Not exactly contradictory but it's an oddity that is never explained.
Shown Their Work: Very often, sometimes to the point of overdoing it. This predominantely shows up whenever cars, ships, historical moments, or diving equipment is mentioned.
Status Quo Is God: In Night Probe, Pitt retrieves an old treaty that proves that the US legally owns Canada. The president uses it as part of a plan to unite the two countries. It never happens. Possibly explained by the fact that in the next book, Deep Six, which takes place a few months later, the same president is kidnapped, brainwashed into acting as a Soviet agent, and impeached after trying to dissolve Congress in order to push a pro-Soviet agenda. This may very well have killed the credibility of his other proposed policies.
Where Does He Get All Those Wonderful Toys?: No one ever explains how Dirk can afford two dozen antique cars and two airplanes on his salary as a senior employee of an obscure government agency. His father's rich, but Pitt doesn't seem like the type of person to blow his trust fund.
Technically, he only had to buy one of the airplanes, and most of the non-car portions of his collection (and some of the car portion) were picked up and/or gifted to him throughout his adventures. Still, the point does stand, since his antique car collection is enormous regardless (it actually parallels the real-life collection owned by Cussler, who used his book sales profits to assemble the collection).
If memory serves, he had received a notable inheritance from either a beloved uncle or grandfather which he uses to buy classic cars at auction. I think it was in the beginning of Dragon.
He inherited a vast amount of money from his grandfather, and as it's hinted he did not live on it (9 days out of 10 he was on assignment in the middle of the ocean) he invested in in the classic cars. It's in Inca Gold, the beginning of Chapter 3 - "The Demon Of Death".
Deconstructed Trope: When Dirk wakes up in a hospital in Iceberg, Sandecker's secretary, Tidi Royal, starts hitting on him. He shuts her down. She protests he doesn't even know she exists, whereupon he reels off her vital statistics—including the location of a mole—and informs her he will never "play games" that close to the Admiral. At the end of the book, Tidi has hooked up with a secondary character, and is never seen again. This explains why James Bond only flirts with Moneypenny.
Flamboyant Gay: Dirk (yes, Dirk) pretends to be one in order to deceive Rondheim.
Out-of-Character Alert: Pitt describes a steak to Kirsti, who has spent most of her life in New Guinea, as being wrapped in echidna seaweed. For those of us unfamiliar with the animal-who who haven't played Sonic the Hedgehog recently—he explains to his friend after Kirsti leaves that an echidna is a type of spiny anteater native to New Guinea. He just said he'd eat the equivalent of a "New York steak wrapped in porcupine quills".
Unwitting Pawn: F. James Kelly initially looks like the book's Big Bad, but he is only a puppet of Rondheim.
Mega Corp.: The Bougainville corporation isn't so powerful as other companies presented in Cussler's books, but it is absurdly efficient, outsmarting both the USSR and the United States government with the president's abduction.
Punch Clock Villain: Aleksei Lugovoy. While he is unambiguously loyal to the USSR and is profiled as quite immoral, he is clearly overwhelmed by the project's magnitude, and doesn't share the degree of fanaticism and meanness shown by his colleague Suvorov.
Reluctant Mad Scientist: Lugovoy is a bad guy, but he would prefer keep his job at the WHO rather than working to overtake the U.S government.
Russian Guy Suffers Most: Among the main two Russian characters, Suvorov is portrayed as the most conventionally evil, and he pays for it.
Defrosting the Ice Queen: It takes weeks of hardship, toil and torture at the hands of the Russians until an escaped Jessie has sex with Pitt under a Cuban bridge.
The Last Dance: Pitt and the sailors sent by CIA are fully determined to move the explosive-laden ships to open water, even if they are 99 percent convinced they will die with them.
Rich Bitch: Jessie LeBaron insults Pitt manifold in just a few hours, until she throws him out of her party, for no logical reason whatsoever. Cue the Oh Crap moment when she hears from the US Secretary of State who is Pitt and how much power NUMA actually has over any marine business.
Shout-Out: The in-universe ruthless President of the USSR Georgi Antonov is clearly based on Yuri Andropov, down to the supposed 1970s plot of Andropov and Mikhail Suslov "staging a tragic accident" by detonating a nuclear bomb.
Germans Love David Hasselhoff: Played straight and dark in-universe with Fidel Castro, when a character says something like this: "we may think him a buffoon, and the Soviets may think the same, but remember, for the common Cubans he is a hero and a god".
Tropes found in Treasure
Tropes found in Dragon
Artistic License - Nuclear Physics: The hidden nuclear devices are the size and shape of a car A/C compressor and powerful enough to blow ships to pieces over dozens of miles and trigger an underwater earthquake. First, to trigger an underwater earthquake megaton-sized weapons are needed, the quoted "maybe 10 to 20 kilotons" is too small, second, only the smallest nuclear devices ever made, W48 and W54 Davy Crockett could fit the size of an automotive A/C compressor, maybe 60x15 centimeters, and their yield was just 72 to 20 tons of TNT respectively, enough to blow to pieces a large ship if hit directly, but never to lay waste to hundreds of square miles.
Also, Pitt's Uncle Percy, retired nuclear physicist, claims the smallest possible nuclear bomb is the size of a baseball. The smallest critical mass of plutonium is a sphere 10.8 centimeters across and it still needs a case, a tamper and a pit, plus detonating devices around.
It's next to impossible to take down the electronics of a large country with multiple nuclear EMPs generated on the ground. The nuclear explosions have to happen in the upper atmosphere to allow the radiation to travel far enough before fading. Otherwise a few thousands of computers and installations may be fried near the explosion sites, but the vast majority of them would be outside the pulse's range.
Simply being on the same ship as a bunch of nukes would not give someone acute radiation poisoning. An undetonated nuke gives off very little radiation. For someone to be as badly affected as the book describes they would have had to do something like take one of the nukes apart and submerge the core in water, or eat bits of it, or something equally daft and improbable.
A Fat Man type nuclear weapon would not be in functional condition after spending several decades under the sea. Even if it had been sufficiently well waterproofed to keep water out of the electrical systems, and they had power to them, the polonium in the initiator (half-life 138 days) would long ago have decayed into uselessness. It would therefore go off with all the alacrity of a wet fart. (Not to mention that the US has many other far more practical systems to deliver a nuke in any case...)
Cool Car: The Murmotos - either a V12 engined sedan of 600 hp, or a 5.8 liter V8 sportscar. In Real Life, the JDM cars are electronically limited to 180 kph and all Japanese sportscars were limited to 280 metric hp prior to 2004 to get through the homologation rules for Japan Grand Touring Championship. Full blown Toyota Supras and Nissan Skylines were for export to US mainland or Europe.
Better to Die than Be Killed: When it looks like the Malians are about to overrun Fort Foreau, Dirk prepares to kill Eva and the rescued women so the enemy doesn't get to rape them. Fortunately, The Cavalry arrives just in time.
Karmic Death: The fate of Massarde. Dirk and Al tie him out in the open until he sunburns, then lets him drink lots of the water tainted by the pollutants he introduced. He dies a raving loon.
Last Stand: Dirk and the UN team have one at Fort Foreau against attacking Malian forces.
Gunn: The State Department experts and the Congressional Committee on Latin American Affairs think you both should hang around and make the dirty Yankees look good by helping to halt the looting of Peru's cultural heritage.
Pitt: In other words, our esteemed government wants to milk our benevolent image for all it's worth.
Badass: Dirk Pitt founds himself on an underground river, battered with critical injuries, in pitch black darkness. He bets his life on the last chip, leaving the river to flow him into the sea over 100 kilometers away, hung by a torn and half-deflated rubber boat. It takes balls to find yourself injured in the very bowels of the Earth and not give up.
Those Wacky Nazis: Let's just say the Wolf family have some serious skeletons in their closet.
Tropes found in Valhalla Rising
Corrupt Corporate Executive: The Big Bad is an oil magnate who wants to sabotage the prototype magnetohydrodynamic drive because of what its mass production would do to the maritime oil business.
Dissonant Serenity: In one scene Al calmly chokes out The Brute who was simultaneously trying to crush him. In contrast, when Dirk was earlier fighting the same guy, he was desperately hurling random stuff.
Would Hurt a Child: The leader of the death squad attacks a plane being flown by Dirk that was conducting a charity flight for sick kids.