Highly enhanced backscattered power levels were measured with EISCAT during a modification experiment with a powerful HF heater. They occurred sporadically when the heater frequency was close to the local plasma frequency and very regularly during a time period when the heater frequency was definitely below the local plasma frequency (overdense heating). In this latter time interval the background electron density in the E region was enhanced due to particle precipitation. From the fact that the enhanced power levels were observed only with the Tromsø antenna looking along the magnetic field line and not with the remote antennas, it was concluded that the artificially created irregularities are pancake-shaped, with the plane of the pancakes perpendicular to the magnetic field. Several processes which may be responsible for the excitation of these irregularities are discussed, and simultaneous results from other ground-based experiments are presented.