  Solitons, or non-destructible local disturbances, are important features of
many one-dimensional (1D) nonlinear wave phenomena, from water waves in narrow
canals to light pulses in optical fibers. In ultra-cold gases, they have long
been sought, and were first observed to be generated by phase-imprinting. More
recently, their spontaneous formation in 1D gases was predicted as a result of
the Kibble-Zurek mechanism, rapid evaporative cooling, and dynamical processes
after a quantum quench. Here we show that they actually occur generically in
the thermal equilibrium state of a weakly-interacting elongated Bose gas,
without the need for external forcing or perturbations. This reveals a major
new quality to the experimentally widespread quasicondensate state. It can be
understood via thermal occupation of the famous and somewhat elusive Type II
excitations in the Lieb-Liniger model of a uniform 1D gas.
