Canopus (Canopus #2)

Helen and her robot companion Arther reluctantly find themselves deep beneath the surface of the mysterious planet, exploring a vast cave system. An impossible encounter with a key figure from Helen's murky past brings the very nature of reality into question, opening a rift between Helen and Arther. As Helen's memories trickle back, their path towards escape grows more and more perilous, leading to a cliffhanger that perfectly fits the definition of 'face-melting.'