Experimental Setup ZULF NMR experimental setup was built/designed by DM technologies. For the relaxation study using NMR at zero magnetic field, the NMR samples, prepolarized for about 20 s inside a permanent magnet (1.4 T), were mechanically shuttled to the zero-field region (interior of a multi-layer magnetic shield) through a guiding field (10 µT) applied by the solenoid (Fig. ). After transfer to the detection region, the samples were stored in the guiding field of the solenoid (storage field) for variable times (from 1 s to 15 s). After that time, the storage field was switched off and an optimal transverse pulse was applied to generate a zero-field NMR signal of maximum amplitude . For the 1 H-13 C spin system, the optimal pulse-induced rotation angle was 4π for the 1 H spins, while in the 1 H-31 P system, the maximum amplitude was achieved using 5π pulse angle for the 1 H spins. To investigate the relaxation of water protons in CuSO 4 solutions, whole blood, plasma, and 15 N-coupled-proton relaxation in N-methylpyridinium solutions, ULF NMR spectroscopy was used (Fig. ). In these cases, after 15 s thermal prepolarization with the 1.4 T permanent magnet, the samples were transferred to the detection region (inside the magnetic shield), where it was stored in various guiding fields (10-510 µT) and times (0.5-6.05 s). Then the guiding field was switched off (within 10 µs) while a detection field of 0.8 µT, corresponding to about 34 Hz of precession of 1 H, was applied in a transverse direction to the sensitive axis of the magnetometer for water-proton relaxation studies. In the N-coupled-proton relaxation measurements, the detection field was adjusted to 1.88 µT (corresponding to 80 Hz of 1 H precession). The detection fields were chosen in such a way that the proton-precession signal appeared in a convenient spectral region, i.e., far from 50 Hz magnetic noise and its overtones but also away from low-frequency flicker noise. It should be noted that a further increase in the solenoid storage field could introduce field inhomogeneities, which may affect the T 2 relaxation. Detailed schematics of our ULF NMR experimental setup and the experimental sequence are shown in Supplementary Information. In the T 1 studies, each data point was obtained after averaging 4-16 transients and is presented with a standard deviation as an error bar. The transverse longitudinal relaxation times T 2 were calculated from the linewidths of the peaks in the spectra. Full maximum half-width (FWHM) values were derived by fitting NMR spectra to the Lorentzian function. Standard-deviation errors of the fitting parameter are denoted with error bars. Data processing and data analysis were performed using custom Python scripts described in Ref. .