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JWST recently measured the transmission spectrum of K2-18b, a habitable-zonesub-Neptune exoplanet, detecting CH$_4$ and CO$_2$ in its atmosphere. Thediscovery paper argued the data are best explained by a habitable "Hycean"world, consisting of a relatively thin H$_2$-dominated atmosphere overlying aliquid water ocean. Here, we use photochemical and climate models to simulateK2-18b as both a Hycean planet and a gas-rich mini-Neptune with no definedsurface. We find that a lifeless Hycean world is hard to reconcile with theJWST observations because photochemistry only supports $< 1$ part-per-millionCH$_4$ in such an atmosphere while the data suggest about $\sim 1\%$ of the gasis present. Sustaining %-level CH$_4$ on a Hycean K2-18b may require thepresence of a methane-producing biosphere, similar to microbial life on Earth$\sim 3$ billion years ago. On the other hand, we predict that a gas-richmini-Neptune with $100 \times$ solar metallicity should have 4% CH$_4$ andnearly 0.1% CO$_2$, which are compatible with the JWST data. The CH$_4$ andCO$_2$ are produced thermochemically in the deep atmosphere and mixed upward tothe low pressures sensitive to transmission spectroscopy. The model predictsH$_2$O, NH$_3$ and CO abundances broadly consistent with the non-detections.Given the additional obstacles to maintaining a stable temperate climate onHycean worlds due to H$_2$ escape and potential supercriticality at depth, wefavor the mini-Neptune interpretation because of its relative simplicity andbecause it does not need a biosphere or other unknown source of methane toexplain the data.

JWST observations of K2-18b can be explained by a gas-rich mini-Neptune with no habitable surface