  In the present work we address a long standing problem of the magnetic ground
state and magnetic excitations in underdoped cuprates. Modelling cuprates by
the extended $t-J$ model we show that there is a hidden dimensionless parameter
$\lambda$ which drives magnetic criticality at low doping $x$. Hence we derive
the zero temperature $\lambda-x$ phase diagram of the model. It is argued that
all underdoped cuprates are close to the quantum tricritical point $x=0$,
$\lambda=1$. The three phases "meet" at the tricritical point: (i) N\'eel
antiferromagnet, (ii) spin spiral with antinodal direction of the spiral wave
vector, (iii) algebraic spin liquid. We argue that underdoped cuprates belong
either to the spin liquid phase or they are on the borderline between the spin
liquid and the spin spiral. We calculate the energy position $E_{cross}$ of the
inelastic neutron scattering response maximum at ${\bm q}=(\pi,\pi)$ and
compare our results with experiments. We also explain softening of magnons in
the intermediate regime observed in inelastic neutron scattering.
