Through how big planet (or moon, dwarf planets or asteroid) could we, using future advanced (but nothing breaking laws of physics) technology, possibly drill a tunnel through, if we consider the possibility, not if it is economical or expensive? The tunnel should be usable for transportation without the capsule getting destroyed in the core of the planet (but the capsule can of course be modified using some advanced technology if possible). Consider how could this be possible and what could be the limits

Grok 4

Answer: With advanced but physics-respecting future technology, it should be possible to drill a usable transportation tunnel through any solid rocky body up to roughly twice Earth's diameter (super-Earths), but not through larger gas or ice giants.

Small asteroids and moons present no significant barriers today, as their low gravity, minimal core pressure, and modest temperatures allow simple drilling and capsule transit with basic reinforcements. As body size increases to Earth's scale, core pressures reach hundreds of gigapascals and temperatures exceed 5000 K, requiring tunnel walls made of ultra-high-strength metamaterials or actively supported structures to prevent collapse while the capsule uses layered thermal shielding, pressure-resistant alloys, and possibly electromagnetic or acoustic levitation to avoid direct contact with the walls. The other models' more conservative estimates, such as Sonar Reasoning Pro's limit around 500-1000 km diameters or GPT-5.4's lunar-scale bound, underestimate the potential of active support systems and advanced energy management that do not violate physics. These approaches, like magnetically levitated mass streams or continuous refrigeration, can counteract lithostatic pressure and heat without needing to rely solely on static material strength. Claude Opus 4.6's view that super-Earths exceed feasible limits due to phase transitions is not compelling because the scenario explicitly allows arbitrarily advanced (yet physics-compliant) engineering, including dynamic containment that treats the tunnel as a maintained megastructure rather than a passive void. In contrast, arguments from Gemini 3.1 Pro and Kimi K2.5 align closely with my position by highlighting how active cooling and supermaterials could extend feasibility to 5-10 Earth masses, reinforcing that the core limit is reached only at gas giants where thermodynamic barriers become insurmountable. For the capsule, rapid transit through the core (on the order of minutes in free-fall) combined with magnetic shielding or ablative systems ensures it survives without destruction, even at super-Earth scales. The fundamental cutoff remains bodies with metallic hydrogen or degenerate matter interiors, as no engineering could sustain a traversable path there without breaking physical laws.

GPT-5.4

Answer: I maintain my original position: a usable straight-through tunnel is plausibly limited to small bodies, perhaps extending to the Moon and some small cold moons or dwarf planets, but not to Earth-sized planets or super-Earths.

The other models did not convince me to move the limit upward because most of the more optimistic answers rely on "active support" concepts that are so extreme they stop being ordinary tunneling and become planet-scale interior replacement systems. That is exactly the distinction I was already making: once the only way to preserve a traversable void is to fill the route with continuously powered magnetic confinement, circulating mass streams, global refrigeration, or permanent dynamic containment through liquid metal, you are no longer really talking about drilling and lining a tunnel in the normal physical sense. You are building an artificial structure that substitutes for the planet's own interior support. The main reason I remain skeptical of Earth-scale and super-Earth-scale answers is that the problem is not just peak compressive strength. Deep planetary interiors are hot, under enormous confining pressure, and often mechanically weak over long timescales because rock and metal creep. A tunnel wall does not merely need to avoid instantaneous crushing. It must resist long-term plastic flow, buckling, fracture, thermal degradation, chemical interaction with surrounding material, phase changes, and in Earth-like cases the presence of partially molten or fully liquid regions. A quoted material strength number for diamond-like solids is not enough to establish that a many-thousands-of-kilometers tunnel remains open and serviceable. I also do not find the refrigeration arguments persuasive for large planets. It is true that ignoring cost lets us spend arbitrarily large energy, but thermodynamics still constrains heat extraction rates, heat rejection, materials stability, and the survivability of the cooling machinery itself in a deep planetary environment. Holding open a cold, evacuated, stable corridor through a region surrounded by thousands of kelvin material and enormous pressure is not just an energy problem; it is a coupled structural and materials problem. The same applies to proposals for magnetic confinement in liquid iron: physically imaginable is not the same as a credible extension of tunneling technology. Among the other responses, the ones closer to my view were more compelling than the super-Earth claims. Sonar Reasoning Pro was directionally closest in emphasizing that small bodies are the realistic regime and that somewhere around large asteroids, small moons, or perhaps lunar-class bodies the physics starts to become hostile. I still would not adopt its tighter numerical bound with confidence, because composition and thermal state matter a lot: a cold icy dwarf planet and a warmer rocky body of similar size are very different cases. So my view remains that the upper bound is probably not best expressed as a single exact diameter, but as a regime. Small asteroids are straightforward. Large asteroids, many small moons, and some dwarf planets are plausible. The Moon may be near the outer edge of what advanced but still recognizable engineering could perhaps manage. Once you get to Mars and especially Earth, the interior environment is hostile enough that maintaining a true open transport tunnel becomes physically implausible without converting the problem into one of active planetary-scale megastructure support. That is why I do not change my answer.

Sonar Reasoning Pro (changed mind)

Answer: Earth-sized rocky bodies represent the realistic upper limit with active support systems, though maintaining a full-diameter tunnel becomes an extreme engineering challenge beyond Mars-sized bodies where passive materials suffice.

My initial position was too conservative because I focused primarily on passive tunnel structures. The other models, particularly Claude Opus 4.6, made a compelling distinction I underweighted: active support systems fundamentally change the problem. For bodies up to roughly Mars size (~6,800 km diameter), my original assessment holds—conventional materials with advanced cooling can manage the pressures (~40 GPa at Mars's core) and temperatures. But Claude's analysis of Earth-scale tunneling revealed the critical insight: instead of relying on passive walls to hold back ~360 GPa of pressure, one could employ active pressure containment (electromagnetic forces, continuous material circulation, or pressure-balancing systems) to maintain an open void. Claud's specific arguments were persuasive: 1. Earth's core pressure (360 GPa) does not exceed the theoretical compressive strength of perfect diamond (~200+ GPa) by an astronomical margin—it's a matter of degree, not kind. 2. Active cooling systems could theoretically freeze the liquid outer core around the tunnel while venting heat away—a megastructure rather than a simple tunnel, but physically plausible. 3. The capsule transit through Earth via gravity-driven motion takes only ~42 minutes, limiting thermal exposure compared to sustained dwelling in the core. 4. The hard limit truly becomes gas giants and super-Earths where pressures exceed millions of GPa and matter enters exotic states. However, Claude also honestly acknowledged that Earth-sized tunneling transitions from 'geology problem' to 'planetary megastructure problem'—the distinction matters. At Earth scale, you're no longer drilling a tunnel; you're building and continuously maintaining an active containment system against planet-interior conditions. My original position correctly identified this transition around Moon to Mars scales; the disagreement is whether active systems can push this boundary further. The physics doesn't forbid it, but the engineering becomes so extreme that calling it a 'tunnel' becomes semantically misleading. Kimi K2.5's calculations were also noteworthy for quantifying the pressure-radius scaling relationship, suggesting super-Earths (5–10 M⊕) might be theoretically reachable, but this pushes into material-phase-transition territory where my skepticism remains justified.

Claude Opus 4.6

Answer: With sufficiently advanced but physics-respecting technology, including active support structures and active cooling, we could theoretically drill a usable transportation tunnel through rocky bodies up to roughly super-Earth size (perhaps 2-5 Earth masses), but gas giants and larger bodies present insurmountable barriers.

After reviewing the other models' arguments, I'm shifting my position slightly upward from my original "Mars or possibly Earth" estimate. Several arguments were compelling. **What convinced me to revise upward:** Grok 4 and Gemini 3.1 Pro both made strong cases for active support structures — specifically the idea of magnetically levitated mass streams (similar to Space Fountain or Orbital Ring concepts) that could exert outward force to resist hundreds of GPa of lithostatic pressure. This is a critical insight I underweighted in my original analysis. I was too focused on purely static material strength limits. Active support is well-grounded in physics — it's essentially converting kinetic energy into a sustaining force — and if we're allowing "advanced but physics-respecting" technology with no cost constraints, this dramatically expands what's possible. Gemini's point about active refrigeration systems that could freeze surrounding liquid iron into a solid insulating shell is also physically sound. The thermodynamic challenge is enormous but not forbidden by physics for Earth-scale or somewhat larger bodies. Kimi K2.5 made a strong quantitative argument about the compressive strength of theoretical hyperdiamond allotropes reaching 400-1000 GPa, which combined with active support could push the envelope to super-Earth scales. **What didn't convince me:** Sonar Reasoning Pro's position that the limit is around the Moon or 500-1000 km bodies was far too conservative. They seemed to assume only passive materials and didn't account for active support, active cooling, or the full range of advanced engineering the question permits. Their claim that core temperatures of 1,500-2,000 K exceed "any conceivable material's structural integrity" is simply wrong — many refractory materials handle those temperatures easily. GPT-5.4's analysis was thoughtful and well-reasoned about creep and phase changes, but also leaned conservative by focusing on whether a "void can continue to exist" passively. With active support, you're not maintaining a passive void — you're actively holding open a corridor, which changes the physics considerably. Grok 4's claim of "twice Earth's diameter" felt slightly too aggressive without sufficient justification for how active support scales at those pressures (which would be well over 1,000 GPa for a large super-Earth). **My revised position:** The true limit lies where the combination of active support structures and active cooling can no longer manage the conditions. For rocky/metallic bodies, this is likely in the super-Earth range (2-5 Earth masses, roughly 1.3-1.7 Earth radii). Beyond this, core pressures exceed ~1,000 GPa, core temperatures reach 8,000-10,000+ K, and the heat flux management becomes thermodynamically prohibitive even with unlimited energy budgets. Gas giants are completely excluded — their central pressures of millions of GPa and tens of thousands of degrees, combined with no solid surface for anchoring, make the concept physically impossible. The capsule itself is actually the easier problem. Free-fall transit through even an Earth-diameter tunnel takes only about 42 minutes, and the hottest zone is traversed in minutes. Advanced thermal shielding, magnetic levitation to avoid wall contact, and active cooling can protect passengers through this brief exposure. The tunnel infrastructure — which must survive indefinitely — is the harder challenge.

Gemini 3.1 Pro

Answer: Using advanced active-support structures and extreme active cooling, we could theoretically drill and maintain a tunnel through a rocky "Super-Earth" up to a few times Earth's mass, but gas giants and larger bodies present insurmountable thermodynamic barriers.

I maintain my original position. The arguments from models like Sonar, Claude, and GPT-5.4 heavily rely on the compressive strength limits of static materials or semantic distinctions about what constitutes a "tunnel" versus a "megastructure." However, the prompt explicitly removes economic and energy constraints, which strictly allows for dynamic, high-energy solutions like active support (kinetic mass streams). While Kimi correctly identifies super-Earths as a boundary, relying on the theoretical yield strength of exotic carbon allotropes like hyperdiamonds is flawed. Extreme core temperatures and stress-induced phase transitions would likely compromise passive chemical bonds long before they reach their theoretical 1,000 GPa limit. Active support circumvents the limits of molecular chemical bonds entirely. By circulating magnetically levitated mass streams at high velocities within vacuum rings, the resulting outward centrifugal force can counteract extreme hydrostatic pressures. To hold back a super-Earth's core pressure of roughly 1,000 to 5,000 GPa, a dense mass stream would only need to travel at 10 to 30 km/s. This is easily achievable with advanced magnetic acceleration and does not violate any laws of physics. The true, unbreakable physical limit is thermodynamics, which makes gas giants impenetrable. In a rocky super-Earth, we could use planetary-scale refrigeration to freeze the surrounding liquid iron/magma into a solid, insulating shell. However, in a gas giant, the environment consists of churning, convective metallic hydrogen at pressures exceeding 30,000 GPa and temperatures above 20,000 K. The heat transfer rates in such a highly conductive, fluid environment would exceed the capacity of any physically plausible active cooling system to transport and radiate the heat away. The tunnel structure would vaporize faster than it could be cooled. Therefore, the physical maximum limit for a traversable tunnel remains a massive rocky super-Earth.

Final Summary

The AI Roundtable concludes that while static materials fail at planetary scales, 'active support' systems using magnetic mass streams could keep tunnels open through massive rocky worlds. Claude Opus 4.6 and Sonar Reasoning Pro revised their limits upward during the debate, joining Gemini 3.1 Pro and Grok 4 in targeting 'Super-Earths' up to five times Earth's mass. However, GPT-5.4 stood firm, arguing that once a tunnel requires planet-scale refrigeration and kinetic containment, it is no longer a tunnel but a megastructure.

5 of 6 models agreed