We aren’t told how long the three brave sisters’ initial dive towards the core of Fundament took, but given that it is a gas giant, we can probably assume that it took days or weeks. During that time, they passed through several layers far beneath their suspended ocean. They encountered vast creatures swimming through the increasingly solid layers of gas and liquid metal. Xi Ro even took some delight in piercing down through a few anemone-like creatures that reminded her of stormjoys from their surface.

They dove and dove until they broke through a layer of metal somewhere deep near Fundament’s core. Here, the needle ship could hear and detect everything. From the slow grinding of the drifting continents, to acid rains that could kill with little warning, to the struggles of creatures as large as cities far far away. It also detected the distant groans and distortions caused by Fundament’s moons lifting up a vast section of Fundament’s inner ocean. Here, perhaps for the first time, Sathona and Xi Ro finally had their own proof that Aurash’s God-Wave was real.

But there was something else down with them. A giant Leviathan that we are told was as large as all the continents they’d known in their childhood before their exile. This massive thing must have been spectacular. We’re told, for instance, that it is propelled by giant fins cracklings with lightning and energy. This Leviathan finds the sisters’ small needle ship and sends powerful bursts of microwave energy booming against its hull… not to attack them but instead to speak to them.

What follows is a conversation between the Leviathan and the three sisters that is split into three parts:

• In the first part, the Leviathan tells them that they live on the edge of a war between the Deep and the Sky. The Sky builds safe places for life to thrive in, it tells them. And it names Fundament, a place of refuge for trillions, as one of those places that the Sky treasures.
  
  But Aurash protests this. She argues that Fundament is no safe place to be treasured. It’s a place of darkness and fatal lightning storms and fiery seas and monsters that hunt her kind from the clouds above. She urges her sisters to continue deeper with her, where she is sure they can discover some greater power to avenge and protect them.
  
  
• Next, the Leviathan asks them what has called them down into the Deep. It tells them that it has been watching their race’s struggle to survive for eons and that their survival and advancement even in such harsh conditions were its proof against despair. It also begins to explain the philosophy of the Deep, but its words are cryptic and hard to understand. It calls the short lived three-eyed krill people its hope, but is telling them that their short lives and hard struggles are right and good...
  
  Xi Ro, of course, protests this. She declares that she will not accept a world where her people’s lives are short and desperate, or a world where a betrayer like Taox can win. She says that she will change the world by killing anything that get in her way.
  
  
• Finally, the Leviathan tries its best to warn the three sisters away from the path that they seem to be on. It admits that yes, the way of the Sky is often harder, but that even though there is suffering and hardship along the way, the Sky is constantly working to build new life and progress towards a gentle world. The Deep, it says, only embraces death. And that if they side with it they will become world killers and will live lives of death and devastation.
  
  Sathona protests this. After years of keeping her father's familiar hidden, she now shows it to her sisters. She tells them how this plain honest worm speaks to her in easy to understand words and that it was the thing that helped her come up with all of her great ideas of the past few years, including helping her find the needle ship.

She then pits the Leviathan and its cryptic ideas against the worm’s plainly stated whispers of hope. Then, Sathona utters what may very well be one of the most frightening and far reaching phrases in all of Destiny. Almost certainly at the urging of her familiar, she says:

Let us dive, oh sisters mine.

We'll see this type of phrase several more time and will come back to its vast importance a bit later. For now, the three brave exiled sisters start their final dive towards Fundament's core and the creatures that have been calling to them for the better parts of their lives...

Sources:
VII: The Dive
VIII: Leviathan

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