The Ringworld Throne by Larry Niven

punched from below. It looked very like Fist-of-God, crowned with vacuum and naked Ringworld floor structure.
    A message from the Hindmost?
    Once the puppeteer knew Louis was moving up the river, he could have moved his probe ahead. He’d sprayed a spy device on this rock cliff, and others elsewhere, no doubt. He’d talked to the Weavers ... easy enough, but why bother? What did he want?
    Something spat from the crater, twice, thrice within ten seconds.
    “Six hundred and ten hours ago,” said a familiar contralto. “Watch.”
    The view zoomed on the three objects. Lens-shaped spacecraft, big. Kzinti design, Louis thought. They stopped just above the peak, then began their descent, two or three meters above the glassy crater wall.
    “The warcraft are moving quite slowly. Let me show you fast-forward,” the Hindmost said. The warcraft moved briskly downslope. Beyond and below, cloudscape jumped into streamlined motion. “In two hours, twenty minutes at just under sonic speed, they had covered fourteen hundred miles. For kzinti, amazing restraint. Then they diverged, thus—“
    The cloudscape and the saucers jerked to a near stop. Two veered off at right angles; the third continued straight on.
    White light blinked. Then the scene was as before, but the three ships had a blobby, half-melted look, and they gleamed like mirrors. They began to descend ... to fall.
    “Stasis fields. They stopped your beam,” Louis said.
    “Worrisome, Louis. Wrong twice within five seconds. Is your brain deteriorating?”
    “That can happen,” Louis said equably.
    The Hindmost said, “Those beams were intense. Vast energy flux was trapped inside the stasis fields before they formed.”
    “But—“
    “You and Nessus survived a similar attack because we design defense mechanisms to react quickly ! Those kzinti warcraft are nothing but bombs now. And that was the Ringworld Meteor Defense, but I did not use it.”
    “Yeah, right.”
    “Observe.” The view jumped ... a view of the magnified sun, darkened to something tolerable. From the fluid storm a plume rose in fast-forward motion. Higher, straight toward the camera ... hundreds of thousands of miles. A brighter shock wave was rising from its base. It lashed out along the plume and was suddenly terribly bright.
    “A superthermal laser effect, definitely the Ringworld Meteor Defense, Louis. But not mine.”
    The Hindmost would lie. But would he shoot down an invading ship?
    “Louis, I’m not shooting down invading spacecraft! I want to contact them. A hyperdrive motor could free me from this place!”
    “I buy that, I guess, but—Hindmost, do you think someone is in the Repair Center with you?”
    “I do not believe my defenses have been breached. Louis, there are two Great Oceans.”
    It took Louis a moment to see what the Hindmost meant.
    A single Great Ocean would unbalance the Ringworld. The water involved would mass as much as a major Jovian moon. There had to be two, on opposite arcs; and there were.
    The Hindmost’s crew had found a Repair Center in one Great Ocean, under the Map of Mars. The other ocean they had never explored at all.
    And it was across the Ringworld’s diameter. The Ringworld was sixteen light-minutes across. Sixteen minutes at lightspeed before a second Repair Center could see invading ships coming through Fist-of-God. Eight minutes more to begin to affect the sun. More time—an hour? Two? -- to stretch a plume of plasma some millions of miles out from the sun, then cause it to lase. The terrible sword of light would be another eight minutes on its way.
    Two hours and twenty minutes was a plausible guess. Louis said, “Stet. You’d best assume there’s another Repair Center on the far side of the Ringworld Arch, and a protector inside.”
    “Why a protector? Mind you, Louis, I think so, too.”
    “A protector would find a way in. If a hominid got in somehow—a breeder—he’ll be a protector by now. The other Repair Center must be infested