This dataset contains some of the raw and preprocessed data presented in the manuscript "Months-long tracking of neuronal ensembles spanning multiple brain areas with Ultra-Flexible Tentacle Electrodes" submitted to Nature Communications. Detailed information on each individual file is as follows: 256ch_device2_impedance_spectroscopy.csv: Impedance magnitudes presented in Fig. 2b. rat1_impedances.csv: Impedance magnitudes belonging to Rat #1 (presented in Fig. 4c). rat2_impedances.csv: Impedance magnitudes belonging to Rat #2 (presented in Fig. 4c). MAX_TBY37_s1_n1_1776_1_12_hires_neuron.tif: Max. intensity projection image of Nissl staining shown in Fig. 4g. MAX_TBY37_s1_n1_1776_1_12_hires_IBA.tif: Max. intensity projection image of IBA staining shown in Fig. 4g. MAX_TBY37_s1_n1_1776_1_12_hires_GFAP.tif: Max. intensity projection image of GFAP staining shown in Fig. 4g. AVG_TBY37_s1_n1_1776_1_12_neuron_4x4bins.tif: z-stack-averaged and binned image of Nissl staining used in histology analysis shown in Fig. 4g. AVG_TBY37_s1_n1_1776_1_12_IBA_4x4bins.tif: z-stack-averaged and binned image of IBA staining used in histology analysis shown in Fig. 4g. AVG_TBY37_s1_n1_1776_1_12_GFAP_4x4bins.tif: z-stack-averaged and binned image of GFAP staining used in histology analysis shown in Fig. 4g. neuron_fluo_ds.npy: The downsampled sample points used in the histology analysis for Nissl staining (Fig. 4g). gfap_fluo_ds.npy: The downsampled sample points used in the histology analysis for GFAP staining (Fig. 4g). iba_fluo_ds.npy: The downsampled sample points used in the histology analysis for IBA staining (Fig. 4g). 256ch_device2_phase_spectroscopy.csv: Impedance phases presented in Supplementary Fig. 7a. rat1_impedance_phases.csv: Impedance phases belonging to Rat #1 (presented in Supplementary Fig. 7b). rat2_impedance_phases.csv: Impedance phases belonging to Rat #2 (presented in Supplementary Fig. 7b). mouseLL2_impedances_magnitudes.csv: Impedance magnitudes presented in Supplementary Fig. 9a. mouseLL2_impedances_phases.csv: Impedance phases presented in Supplementary Fig. 9b. mouseLL2_single_unit_SNRs.csv: Single unit SNRs presented in Supplementary Fig. 9c. mouseLL2_single_unit_lifetimes.csv: Single unit lifetimes presented in Supplementary Fig. 9d. Figure_5_data.mat: Data used in Figure 5 (can be imported into the corresponding Matlab script in the GitHub repository). The rest of the data supporting the figures is provided in the Source File and Supplementary Data files, which are available through the online version of the article. Any additional requests for information can be directed to, and will be fulfilled by, the corresponding author.