Contained Calibrated Atmospherical Nearly-Non-Magical Sub-Aquatic Transport and Conveyance Device

The gnome engineer Kadeem Al-Jazarah had a modest workshop near the docks in Itatlan. When not obsessing over a new invention, he would walk along the jetties and watch the ships as they departed for and returned from the ports around the Roaring Lake. The fortnight of terrible storms and powerful earthquakes had damaged much of the city. The intravening week had seen a flurry of work to clear the streets and make essential repairs, and as many of the dock crews had been requisitioned for labor, the docks were unusually empty and quiet. Which is why Kadeem was the first surfacer to meet some very new arrivals.
During and after the cataclysm, maritime trade had been suspended, save when absolutely essential. Though the waters were now safe, few of the large barques had set sail, many of them damaged. Some smaller boats had come and gone, and every day more and more fishermen did as well. The taverns were filled with news from around the lake, of how the various cities and towns had weathered the cataclysm, of forests leveled, of mountains collapsed, of new rivers. But also of the many strange, unknown - and sometimes alien - creatures washed up on the shores. Boatmen spoke of strange shapes and noises encountered within the mists of the lake, and the old and wise nodded gravely at these portents.
Kadeem had heard much of this, and though he prided himself on his creativity and imagination, he was nonetheless completely unprepared to see, silently emerging from the mist, a hundred foot-high fish, its jaws opened wide and carrying within them a host of fish people. Fortunately the locatha came in peace, and Kadeem became their first interlocutor. The story of their first meeting with the Yoxchopan Jade Queen is a story for elsewhere, but during the locatha delegation's stay Kadeem was indispensable, and from them he learned much about the bizarre world beneath the earth. More than anything, he wished to see it with his own eyes.
When the delegation departed, Kadeem locked himself in his workshop. Not even his assistants saw him - they passed him food beneath his study's door, and he passed them notes for strange materials and instructions for building even stranger objects. After three months of this, Kadeem Al-Jazarah emerged (thinner, paler, and sporting a fantastic new mustache) with blueprints and a complete working miniature of the invention which would bring him to the deeps: the Contained Calibrated Atmospherical Nearly-Non-Magical Sub-Aquatic Transport and Conveyance Device.
Most people just call them sub-aquatics.

Utility

Sub-aquatics are primarily used by terrestrial merchants to ferry goods to and from aquatic cities. While more expensive upfront to buy than an enchanted ship, and having less stowage, they require less crew, are easier to navigate and repair, and can travel nearly twice as fast.
They are also used by many governments to send official representatives. Very occasionally an extremely wealthy or influential individual will hire one out for exploration or sightseeing.

Access & Availability

The precise production secrets of sub-aquatics are a closely guarded gnomish secret, though as with most other gnomish inventions, the secrets are shared between gnome communities. As a result, gnome drydocks have sprung up in most cities on or near passages leading to the aquatic realms.