We present an investigation of a single sunspot observed in the neutral Fe line at 1089.6 nm with the Tenerife Infrared Polarimeter at the Vacuum Tower Telescope on Tenerife. Using the SIR code, we obtain maps of the magnetic field strength, inclination and azimuth, and Doppler velocities. The magnetic field strength drops from 2800 G in the umbra to about 700 G at the outer penumbral boundary, where we encounter an average magnetic inclination of 72o. Comparing the magnetic flux passing through different areas, we conclude that the inner penumbra must be deep, while the outer penumbra could be shallow. Assuming that the magnetic field strength encountered at the outer penumbral boundary forms a smooth surface through which the total flux of the spot passes, it would be approximately an ellipsoidal cap with a top height of 5250 km. This scenario leads to an average vertical magnetic gradient of 0.4 G km-1. Evershed Doppler velocities are about 2 km s-1. Two penumbral locations related to dark intensity features exhibit a steeper and slightly stronger magnetic field than elsewhere in the penumbra, and one of them is connected to an interruption of the Evershed effect.