High and ultrahigh energy neutrinos provide a unique window into the most extreme environments in the Universe. Produced in powerful cosmic accelerators such as active galactic nuclei, these elusive particles traverse vast cosmic distances and reach Earth unharmed by magnetic fields or matter. In this talk, we explore how neutrinos act as messengers from supermassive black holes, revealing the physical conditions and processes in their vicinity. We also consider how their observation, in combination with other messengers, may provide subtle clues to the intimate nature of gravity at the quantum level. Highlighting recent results from IceCube and KM3NeT, I will present the scientific vision behind the Pacific Ocean Neutrino Experiment (P-ONE), which not only aims to deepen our understanding of the high-energy universe, but also promises to become a unique infrastructure for studying the rapidly changing ocean environment.

Among all known isotopes, Thorium-229 has the lowest nuclear excited state, only 8.4 eV above the ground state. This so-called “isomer” is accessible to VUV laser excitation and a multitude of applications at the interface of atomic and nuclear physics have been proposed, including a nuclear clock, a gamma laser and a sensitive detector for variations of fundamental constants. After decades of attempts to determine the exact isomer energy and other nuclear properties, we report on two experiments which resonantly excite the isomer by lasers and spectroscopically resolve the nuclear quadrupole splitting in a single crystal environment. This allows us to determine the sensitivity of the nuclear clock transition to variations of the fine structure constant, which exceeds all schemes involving electron shell transitions by 3 orders of magnitude.