To find the way
Despite its strong resonance and fairly great size, Ohllm have only been discovered as of late. This is the consequence of two distincts, but linked, causes :
The seamark is located in shallow waters, just under the sky. According to the few knowledgeables that accessed it, if it was a little bit higher the rapid currents would have disrupted the frail equilibrium that allowed the zone to form. Only the existence of a skip able to rise to the Clouds made it possible to reach the viscinity of Ohllm.
Some believe that Ohllm is formed by the interferences of the very odd resonance of Clouds. They undoubtebly emit something, but not translatable in any dialect, not even in Unitone. And the seamark is constantly moving in order to always stay under an area poor in Clouds. Since their signals don't overlap at this point of the ocean, they interact and the result is Ohllm, as stated by this theory. A claim strengthened by the occasional disappearance of the signature, which then reappear at a great distance.
Due to these oddities, it wasn't discovered until the Revealer ventured in the shallows. Amazed by this display of forces beyond understanding, he aimed to unravel the mystery of Ohllm. Some says it was his first and foremost trial of knowledge, the one that left him with an inextinguishable thirst for answers. Whether that is true or not, he ended up with a bittersweet taste. At this time he wasn't able to come upon the truth, and was terribly frustrated.
While he wasn't yet content with his findings, he didn't give up and settled Ohllm as the meeting point of the nascent Seeker's organization. A choice that turned out to be wise, as they continued to unveil the shroud of mysteries.
A most perfect hideout
Ohllm has the shape of a gigantic bubble, large enough to engulf a medium-sized Sphere and still have some space left. This contour is only assumed, as the water doesn't change in any noticeable way. The only way to delimit the area is to enter, because the plangency is not felt inside.
The resonance of Ohllm is very strong, albeit limited in range. So much that it covers the signature of any skip, even though they may have a wider, but fainter, scope. In an era where the skips are identified from very far away by their unique signature, any skip that seeks refuge in this zone become virtually invisible to any pursuer. This, combined with conditions unsuitable for most regular skips, designed the seamark to be the prefered place for the Seeker's meeting.
A place of legend
Ohllm is not the least cause in the myth of the elusivity of Anhvall and the Seekers. More than one hunter who was tailing an agitator's crew from far away to suddenly lose every clue and be left speechless. Even after word got out about the secret hideout of the organization, a clever propaganda transformed the site into a dreaded fortress in the mind of people. Somewhere between a Sphere and a swarm of armored skips, everyone believes the place to be a most wonderful opponent. When the Righteous Order or the Divine League look for the legendary Ohllm, they send powerful armadas in search of an even match, always in vain except for one time.
The assault of Ohllm
Once, the shelter was in jeopardy. It wasn't discovered until later how the Righteous Order found out the location of Ohllm. Treason was considered, but the truth was laughably simple. A fake Anhvall was captured, and its crew tortured to death without spilling any information. But then their persecutors found an exotic device, a cylindrical compass.
How to tone down a signature
Usually, the stronger the vibration, the wider the range at which it can be sensed. Other factors play a role too, such as the pressure or the temperature. Waves travel slower in cold waters and at low pressure. But this alone shouldn't be able to explain the short range of Ohllm's signature.
A signal this powerful should be heeded across at least half of the Expanse. The explanation behind this bizarre fact lie in the pattern of the signature. Ohllm. A motif so simple it quickly fades in the natural resonance of the water and become unrecognizable. Hence why nobody ever crossed the place for the first time with a purpose : who would look for what seems to be a most ordinary wave formed by any current ?
Though it gets mixed up with the natural resonance, there is a very subtle way to trace back the signal. In the shallows, the initial signature sounds like "Oooolllm". Tracking the shorter patterns which deviates ever so slightly from the main tone cannot be done without knowing that this pattern exists in the first place. And even then, only a trained ear is able to identify the origin and reach Ohllm.
The Seeker's Compass
In their earlier days, gathering at Ohllm was a real pain for the young Seekers, and many got lost in the process. This was the case until the Revealer himself created the Seeker's Compass. An artifact in the shape of a cylinder, with a cleft on both faces and several barrels containing water echoing the signature of Ohllm in the manner of a speech bubble. When put in the Expanse, the most sensitive barrels can grasp even the faintest resonance, and will emerge from the cylinder to align their phases. The closer the compass is from Ohllm, the stronger the signal and the barrels arise one row after the other. When the last of the six rows have come out, the skip should be close enough for the skipper to hear by themselves the resonance.
In later cycles, the Revealer decided to provide his invention for the wide public, eventually coming up with a way to revolutionate seafaring. Instead of lying on the sense of great signature, compasses only need water imbued with a slight resonance to lead anywhere in the Expanse. It goes without saying that this provoked a great turmoil as this innovation had been suppressed for long up until this time.
Multiple Ohllms ?
Considering the assumed circonstances of the appearance of Ohllm, it doesn't seem far-fetched to imagine the existence of multiple instances of the phenomenon taking place at the same time. However, the Seeker's Compasses only point to a single location, and never was a skip misguided to a different Ohllm than the one it aimed.