Magnetic monopoles emerge as emergent quasiparticles in spin ice, a class of exotic magnets. Spin ice materials have magnetic moments on a pyrochlore lattice, constrained to point along local Ising axes. These moments can be modeled as Ising spins, and the system is described by an energy term involving nearest-neighbor exchange and long-range dipolar interactions. Spin ice does not order at low temperatures, exhibiting residual entropy similar to ice, due to the ice rule, which requires two spins pointing in and two out at each diamond lattice vertex. Excitations above the ground state, which violate the ice rule, are identified as magnetic monopoles with long-range properties. These monopoles can be separated and interact via a magnetic Coulomb law, leading to a liquid-gas transition in the magnetic monopole density under an applied magnetic field. The monopoles are deconfined and exhibit genuine magnetic interactions, with their charge magnitude being much smaller than the fundamental magnetic charge. The presence of magnetic monopoles in spin ice is supported by experimental observations of a liquid-gas transition and a critical endpoint. The phase diagram of spin ice in a [111] magnetic field shows a first-order transition at low temperatures, terminating in a critical point. This transition is distinct from other systems and is explained by the deconfined monopoles. The monopoles are not quantized in the same way as fundamental magnetic monopoles, as their interactions are entropic rather than magnetic. The study provides a natural explanation for the observed phase transitions in spin ice and highlights the importance of magnetic monopoles in understanding the behavior of strongly interacting many-body systems. The results suggest that spin ice is a rare example of high-dimensional fractionalisation, with implications for fields such as correlated electrons and topological quantum computing. The findings encourage experimentalists to directly detect these monopoles through various methods, including scattering, transport, and noise measurements.Magnetic monopoles emerge as emergent quasiparticles in spin ice, a class of exotic magnets. Spin ice materials have magnetic moments on a pyrochlore lattice, constrained to point along local Ising axes. These moments can be modeled as Ising spins, and the system is described by an energy term involving nearest-neighbor exchange and long-range dipolar interactions. Spin ice does not order at low temperatures, exhibiting residual entropy similar to ice, due to the ice rule, which requires two spins pointing in and two out at each diamond lattice vertex. Excitations above the ground state, which violate the ice rule, are identified as magnetic monopoles with long-range properties. These monopoles can be separated and interact via a magnetic Coulomb law, leading to a liquid-gas transition in the magnetic monopole density under an applied magnetic field. The monopoles are deconfined and exhibit genuine magnetic interactions, with their charge magnitude being much smaller than the fundamental magnetic charge. The presence of magnetic monopoles in spin ice is supported by experimental observations of a liquid-gas transition and a critical endpoint. The phase diagram of spin ice in a [111] magnetic field shows a first-order transition at low temperatures, terminating in a critical point. This transition is distinct from other systems and is explained by the deconfined monopoles. The monopoles are not quantized in the same way as fundamental magnetic monopoles, as their interactions are entropic rather than magnetic. The study provides a natural explanation for the observed phase transitions in spin ice and highlights the importance of magnetic monopoles in understanding the behavior of strongly interacting many-body systems. The results suggest that spin ice is a rare example of high-dimensional fractionalisation, with implications for fields such as correlated electrons and topological quantum computing. The findings encourage experimentalists to directly detect these monopoles through various methods, including scattering, transport, and noise measurements.