Passive RC network with voltage gain greater than unity?
To put in the simple term, the phase shift created by R1, C1/C2 and the phase shift created by R2, C2 are as such that the phases of voltages across C1 and C2 are aiding each other (although not perfectly in the same vector direction), their combined value is greater than the supplied voltage. 

This is not so extraordinary as there are some op amp circuits especially notch filters which uses resistors and capacitors to stimulate the behaviour of inductors.