to reduce the physical stress of g forces. forces applied to fluids are distributed as omnidirectional pressures. because liquids cannot be practically compressed, they do not change density under high acceleration such as performed in aerial maneuvers or space travel. a person immersed in liquid of the same density as tissue has acceleration forces distributed around the body, rather than applied at a single point such as a seat or harness straps. this principle is used in a new type of g-suit called the libelle g-suit, which allows aircraft pilots to remain conscious and functioning at more than 10g acceleration by surrounding them with water in a rigid suit. acceleration protection by liquid immersion is limited by the differential density of body tissues and immersion fluid, limiting the utility of this method to about 15g to 20g. extending acceleration protection beyond 20g requires filling the lungs with fluid of density similar to water. an astronaut totally immersed in liquid, with liquid inside all body cavities, will feel little effect from extreme g forces because the forces on a liquid are distributed equally, and in all directions simultaneously. however effects will be felt because of density differences between different body tissues, so an upper acceleration limit still exists. liquid breathing for acceleration protection may never be practical because of the difficulty of finding a suitable breathing medium of similar density to water that is compatible with lung tissue. perfluorocarbon fluids are twice as dense as water, hence unsuitable for this application. examples in fiction literary works alexander beliaev's 1928 science fiction novel amphibian man is based on a scientist and a maverick surgeon, who makes his son, ichthyander (etymology: "fish" + "man") a life-saving transplant – a set of shark gills. there is a film based on the novel. l. sprague de camp's 1938 short story "the merman" hinges on an experimental process to make lungs function as gills, thus allowing a human being to "breathe" under water. hal clement's 1973 novel ocean on top portrays a small underwater civilization living in a 'bubble' of oxygenated fluid denser than seawater. joe haldeman's 1975 novel the forever war describes liquid immersion and breathing in great detail as a key technology to allow space travel and combat with acceleration up to 50 g. in the star trek: the next generation novel the children of hamlin (1988) the crew of the enterprise-d encounter an alien race whose ships contain a breathable liquid environment. peter benchley's 1994 novel white shark centers around a nazi scientist's experimental attempts to create an amphibious human, whose lungs are surgically modified to breathe underwater, and trained to reflexively do so after being flooded with a fluorocarbon solution. judith and garfield reeves-stevens' 1994 star trek novel federation explains that before the invention of the inertial dampener, the stresses of high-g acceleration required starship pilots to be immersed in liquid-filled capsules, breathing an oxygen-rich saline solution to prevent their lungs from being crushed. nicola griffith's novel slow river (1995) features a sex scene occurring within a twenty cubic foot silvery pink perflurocarbon pool, with the sensation described as "like breathing a fist". ben bova's novel jupiter (2000) features a craft in which the crew are suspended in a breathable liquid that allows them to survive in the high-pressure environment of jupiter's atmosphere. in scott westerfeld's sci-fi novel the risen empire (2003), the lungs of soldiers performing insertion from orbit are filled with an oxygen-rich polymer gel with embedded pseudo-alveoli and a rudimentary artificial intelligence. the novel mechanicum (2008) by graham mcneill, book 9 in the horus heresy book series, describes physically crippled titan (gigantic war machine) pilots encased in nutrient fluid tanks. this allows them to continue operating beyond the limits normally imposed by the body. in liu cixin's novel the dark forest (2008), the warships of humanity in the 23rd century flood their compartments with an oxygen-rich liquid called 'deep-sea acceleration fluid' to protect the crew against the forces of extreme acceleration that the ships undergo. ships enter a 'deep-sea state' where the crew are immersed in the fluid and sedated before acceleration can commence. in the 2009 novel the lost symbol by dan brown, robert langdon (the protagonist) is completely submerged in breathable liquid mixed with hallucinogenic chemicals and sedatives as a torture and interrogation technique by mal'akh (the antagonist). he goes through a near death experience when he inhales the liquid and blacks out, losing control over his body, but is soon revived. in greg van eekhout's 2014 novel california bones, two characters are put into tanks filled with liquid: "they were given no breathing apparatus, but the water in the tank was rich with perfluorocarbon, which carried more oxygen than blood." in author a.l. mengel's science fiction novel the wandering star (2016), several characters breathe oxygenated fluid during a dive to explore an underwater city. they submerge in high pressure "bubbles" filled with the perfluorocarbon fluid. in tiamat's wrath, a 2019 novel in the expanse series by james s. a. corey, the laconian empire utilizes a ship with full-immersion liquid-breathing pods that allow the crew to undergo significantly increased g-forces. as powerful and fuel-efficient fusion engines in the series have made the only practical limitations of a ships' acceleration the survivability of the crew, this makes the ship the fastest in all of human-colonized space. films and television the aliens in the gerry anderson ufo series (1970-1971) use liquid-breathing spacesuits. the 1989 film the abyss by james cameron features a character using liquid breathing to dive thousands of feet without compressing. the abyss also features a scene with a rat submerged in and breathing fluorocarbon liquid, filmed in real life. in the 1995 anime neon genesis evangelion, the cockpits of the titular mecha are filled with a fictional oxygenated liquid called lcl which is required for the pilot to mentally sync with an evangelion, as well as providing direct oxygenation of their blood, and dampening the impacts from battle. once the cockpit is flooded the lcl is ionized, bringing its density, opacity, and viscosity close to that of air. in the movie mission to mars (2000), a character is depicted as being immersed in apparent breathable fluid before a high-acceleration launch. in season 1, episode 13 of seven days (1998-2001) chrononaut frank parker is seen breathing a hyper-oxygenated perfluorocarbon liquid that is pumped through a sealed full body suit that he is wearing. this suit and liquid combination allow him to board a russian submarine through open ocean at a depth of almost 1000 feet. upon boarding the submarine he removes his helmet, expels the liquid from his lungs and is able to breathe air again. in an episode of the adult swim cartoon series metalocalypse (2006-2013), the other members of the band submerge guitarist toki in a "liquid oxygen isolation chamber" while recording an album in the mariana trench. in an episode of the syfy channel show eureka (2006-2012), sheriff jack carter is submerged in a tank of "oxygen rich plasma" to be cured of the effects of a scientific accident. in the anime series aldnoah.zero (2014-2015), episode 5 shows that slaine troyard was in a liquid-filled capsule when he crashed. princess asseylum witnessed the crash, helped him to get out of the capsule, then used cpr on him to draw out the liquid from his lungs. video games in the classic 1995 pc turn-based strategy game x-com: terror from the deep, "aquanauts" fighting in deep ocean conditions breathe a dense oxygen-carrying fluid. in the eve online universe (2003), pilots in capsules (escape pods that function as the control center for the spacecraft) breathe an oxygen rich, nano-saturated, breathable glucose-based suspension solution.