Ultrahigh energy (UHE) astrophysical neutrinos that originate in interactions of cosmic rays with the microwave background radiation, or from astrophysical sources such as Active Galactic Nuclei (AGN) and Gamma Ray Bursts (GRBs), provide a unique way of studying particle physics as well as astrophysics.  I will discuss what we can learn from UHE cosmic neutrinos. I will show that the effect of neutrino oscillations is production of tau neutrinos which can provide an enhanced signal for the detection of cosmic neutrinos. Furthermore, interactions of UHE cosmic neutrinos could potentially lead to the production of microscopic black holes predicted in theories of extra dimensions, or they may produce supersymmetric charged particles that travel large distances, such as staus. I will discuss these processes and their signals in neutrino detectors such as Anita, IceCube, EUSO and OWL.