Silicon electrode arrays with thousands of channels enable read-out of the activity of large populations of neurons, but they induce chronic inflammation in the brain and a decline in signal quality. On the other hand, existing flexible electrode arrays have limitations on how reliably they can be inserted into any targeted depth in the brain while maintaining a high density of recording channels. To address these problems, we developed Ultra-Flexible Tentacle Electrodes (UFTE), where each recording channel is mechanically independent to provide maximal compliance with brain tissue—while also allowing the packing of many channels in the smallest possible footprint. UFTEs can be implanted in parallel at a low speed of 12.5 µm/s without limitations in implantation depth, thanks to a novel combination of mechanical- and chemical-tethering during insertion. We also devised a scheme to implant UFTEs simultaneously into many brain areas at arbitrary locations without angle-of-insertion limitations, with hundreds of channels. Spike SNRs were 1.5-3x compared to the state-of-the-art. We achieved simultaneous recordings of stable single-units and ensemble activity from medial prefrontal cortex, retrosplenial cortex, and hippocampus in freely moving rats while tracking some units putatively for up to 3.5 months. Recordings from mouse hippocampus were stable for nearly a year (longest duration tested). Immunostaining of brain slices revealed no detectable chronic damage caused by the electrodes to surrounding brain tissue. We are expanding our electrode arrays and recording electronics to encompass a broader range of brain areas, enabling wireless and untethered recordings with a higher number of channels.