Ancient Atlantis possessed truly advanced technology, including energy weapons, force fields, and aircraft, among other things. However, this technology was made up of a combination of scientific principles, the alien science and technology of the Preservers, and Atlantean understanding and use of magic, including ley-line energies, enchanted crystals, and tapping into various elemental and geomantic forces (some of which may have weakened the strata and geological stability of the island-nation).
Some Atlantean technology survived the Sinking and the subsequent millennia immersed in icy salt water, and it has led to significant differences between what Atlanteans and surface people consider “technology”. In particular, while some of their weapons, vehicles, and even household elements may appear “high-tech,” Atlantean technology does not rely upon electronics or electricity (virtually useless underwater unless completely sealed), nor upon combustion or the combination of volatile chemicals (also problematic underwater).
Instead, most Atlantean technology is powered by magical crystals drawing upon geomantic and elemental energies, recharged in orichalcum-lined sockets linked to ley-lines, or from the user's own personal stores of mystical energy. So while an Atlantean blaster-weapon may look like a pistol or a rifle, a surface world engineer taking one apart would find it a technological impossibility. The same is largely true of Atlantean vessels and other devices, such as the occasional Atlantean audio-visual projector, actually a crystalline-based image generator, often with some telepathic components, rather than a video screen or camera.
The nature of Atlantean mechano-mystical devices makes their technology less mass-produced and ubiquitous compared to the surface world, since each item is hand-crafted by specially-trained experts. Indeed, many are often family heirlooms maintained, repaired, and passed down through the generations, dating back to before the Sinking, patched together with whatever components were available -- re-creating such items are simply beyond the capabilities of modern Atlanteans. This leads to an apparent mixture of ancient and modern styles and resources in Atlantis and a somewhat “retro-future” quality to their technology, since it generally lacks things like computers and electronics. (Picture sci fi stories from the 1920s-1950s, before transistors and integrated circuits.)
USES AND LIMITS OF ATLANTEAN TECHNO-MAGIC
Atlantean Technology/Techno-Magic is very good at what would be considered alchemy (though now very limited due to their aquatic environment), as well as effects that involve or manipulate the four "classical elements": air, earth (and metal), fire (and heat), and water (including cold). Fire itself is not often used, since that doesn't work underwater, but heat is. It is also very good at generating and manipulating light (and darkness) and sound, which are considered "lesser elements." Electricity & lightning are known, but rarely used, in part due to the problems in using it underwater, and in part because of ongoing debates on which element it should fall under. Plants can be manipulated indirectly, by manipulating the minerals and water around & within them.
Many forms of Atlantean Tech have been developed that channel the power of Law/Order -- bonds or large mazes of mystic force, locking shapeshifters to one form, repairing/restoring broken items, sealing dimensional breaches -- developed to counter their Chaos-using Deep One enemies. And as Lemurian techno-mages have proven, similar devices can also be constructed to channel the powers of Chaos.
Body-altering magics are rare. Their last widespread use was in altering survivors of the Sinking to be able to breathe water. (Some Atlantean sorcerers engage in shapeshifting battles, but they're viewed as... weird.) Healing devices are available, which involve some restorative Law/Order magics, but are not nearly as common as they once were and so usually reserved for extreme cases. The techno-magical equivalent of bionics/cybernetics are possible, but rare, and usually done to replace & replicate the function of a lost body part, not to enhance or add new abilities. Some Atlantean athletes and soldiers use performance-enhancing potions or ointments, but they're viewed with disdain, not for any harmful side-effects but because they should be able to rely on their own skills and aptitudes. The taboo against body-altering magics is largely born of anti-Deep One sentiment, fearing the polymorphic power of the Serpent Scepter, and the protean nature of their otherworldly masters.
For similar reasons, and also out of respect to the Sirens, Mental effects are rare. Devices permitting telepathic communication -- between Atlanteans and other sapients, and Atlanteans and animals -- are common, as are illusions, though most are obviously illusions and so suitable only for entertainment or educational displays, not deceit. Crystalline devices that can record and play back thoughts are also common, and are one of the few pre-Sinking items that can be recreated. Emotion control is accepted, if tied to the elements (via the "four humours"), but mind reading and mind control -- even controlling non-sapient animals -- are deeply feared, and is something only those vile Lemurians would do. (The sea creature-controlling Trident of Poseidon gets a pass due to its use in defending Atlantis over the centuries.) Order-focused artificers argue for being able to develop truth-compelling effects, to cut through the chaos of lies and deception; debates are still ongoing, though rumors persist of "secret police" who do employ such techniques. (In truth, Atlantean techno-magic is just as capable of generating and manipulating psionic energies as any other form of elemental energy, though this is not widely known.)
Atlantean Tech is capable of teleportation across space, but only using large platforms connected to ley lines & only to teleport to other such platforms. Experiments with using charged crystals to power more mobile teleporters have met with limited success. (Dimensional and temporal travel are theoretically possible, but would require more complex calculations and even larger amounts of energy.) Flying vehicles are common, but wearable "flight packs" are still being worked on.
Atlanteans were adept at creating magiconstructs -- automatons in humanoid or animal form, serving a variety of military and construction functions -- but the techniques to create them are long lost. Some Atlantean statues -- on the isle or in surface world museums -- are actually long-dormant colossi, their inner furnaces long depleted of fuel.
Though Atlantean technology is as much magic as it is science, it is not used to invoke or entreat any mystic entities, even ones who are called on for elemental effects (like Abbridon, Ghorummaz, or Phoros), though Atlantean sorcerers can and regularly do. The reasons for this are twofold: they don't want to become beholden to any outside entity (as the Deep Ones are to the Unspeakable One), and the Preserver tech components have trouble with specific mystical entities of such great power. Their techno-magic can be used to summon elemental beings, using large arcane patterns developed over millennia and altered based on what needs to be summoned, and an ample supply of elemental material to form the body, though whether these "spirit matrices" actually summon spirits from some other plane, or creates something like an A.I. molded by the user's subconscious, is a matter of some debate.
Atlantean Techno-Magic is capable of necromantic effects -- manipulating both life force and energies from der Shattenwelt are within its capabilities -- but such devices are extremely rare, and largely relegated to dark myths.
The ancient Atlanteans invented a unique metal alloy of copper, gold, mercury, and silver, called orichalcum, a hybrid creation of advanced metallurgy and alchemy. (Surface world metallurgists get confused and infuriated when examining it, because such an alloy should not be possible.) The secret of its creation was lost in the sinking of Atlantis, and likely would not be practical in an aquatic environment, but samples of the metal still exist, owing to its near-indestructible nature, and Atlantean “cold-forges” can reshape existing orichalcum to a degree. Orichalcum is a golden (or deep coppery) metal with Toughness 25 and immunity to damage from temperature extremes. (If put in a fire, when removed it won't be any hotter and can be safely handled with ungloved hands; if left in a snowbank, when removed it won't be any colder.) Also, despite being largely unaffected by magic -- magical energies pass through, flow around, or are absorbed into it -- it holds enchantments better than most any substance, making it ideal for crafting magical armor, weapons, vehicles, and other devices. Orichalcum weapons, armor, and tools are prized heirlooms among the Atlanteans (and their Utopian cousins).
Atlantean mechano-mystics also recognize four distinct elemental substances, though these are so basic a component of their craft that they rarely bother describing them to anyone. Corusqua are incandescent blue crystals which contain electricity, though they are inert unless agitated by mystical energy; their use became much less common post-Sinking, though some have been repurposed into remote- or proximity-detonated area effect weapons (i.e., mines). Crystallos is a transparent crystal similar to quartz, but grows like a plant; it can be constrained and forced to grow into various shapes (typically flat panes), dyed by soaking in pigments, and can be baked in special arcane furnaces to halt their growth but make them harder than steel (though much lighter). Fulminor is a blue-black smoke that is lighter than air, and can impart that quality on any substance it remains in extended contact with; it is used in the fuselages of Atlantean aircraft. Ignaetium is a pale blue, dull orange, or blood red crystal, containing a spark of Elemental Fire, and is used to fuel many arcane furnaces & as a source of heat. All four are naturally occurring substances, but can also be created in a sorcerer-scientist's workshop.
As for mundane substances, copper -- and its alloys bronze and brass -- are often used in Atlantean metallurgy, due to their availability and unreactiveness to most mystical energies. (Atlantean writing is done by die-stamping onto thin sheets of such soft metals.) Iron and most steels rust in the saltwater environment, though they do have various sealants which can slow that corrosion. The alchemical resonances in gold and silver tend to interfere with the mystical elements of their devices if present in large quantities, but are sometimes used in small quantities for decorative purposes.
Among other things, the differences in Atlantean technology impose a +20 DC modifier to Knowledge (Technology) skill checks dealing with it, unless the character is also trained in Knowledge (Arcane Lore), in which case the lesser of the two skills may be used without the +20 DC modifier. Similarly, repairing or constructing Atlantean technology requires training in both Craft (Mechanical) and either Craft (Artistic) or Craft (Structural). Surface-world trained technicians can barely make heads or tails out of Atlantean techno-magic.
In a like manner, modern electronics, or even simple combustion engines, are just as weird and alien to Atlanteans as their techno-magic is to surface worlder engineers. An Atlantean trying to deal with Surface world technology suffers a +20 DC modifier to their Knowledge (Technology) checks, unless they are also trained in Knowledge (Physical Science), in which case the lesser of the two skills may be used without the +20 DC modifier.