4 ideas
The aliens evolved inside a spherical cavity enclosed by solid rock, and they took that rock to be the whole universe. Explorers tunnelled outward through the stone, sometimes at great cost, and eventually broke through the planet's surface. That is how they discovered that their 'universe' was a hollow inside a planet floating in space under a sky of stars.
An alien ship hovering over the ocean pulls the sea upward with its gravity, forming a water mountain thousands of metres high. Feng Fan is a disgraced climber who had sworn off mountains after a fatal expedition. He swims and climbs to the summit of this water mountain, and from its peak he talks with the aliens about their origins.
The story argues that the impulse to go up and out, 'because the mountain is there', is not a human quirk but a defining trait of any mind. A civilization keeps its vitality only while it keeps climbing toward the next boundary. A civilization that stops climbing has effectively ceased to be what it was.
Observers inside a bounded world naturally treat its walls as the limits of reality, because nothing beyond the walls sends them evidence. Their assumption is only overturned by physically pushing through the boundary, not by thinking harder about the enclosed space. This invites us to ask which of our own 'solid rock' assumptions are really just the limits of where we have dug.