||IRIS Bombs (IBs) as an important class of UV bursts, might be associated with U-loops, EBs and FAFs. Therefore, they might have different magnetic geometry and thus occur in different plasma environments, and this could be a reason that different IBs might form at different heights. A question is whether IBs haveing similar magnetic geometry form in similar plasma environments. To this end, 12 IBs in four IRIS data sets, located in the combined foot-points of multiple transition region loops, are investigated in this study. Likely, these IBs fit with the U-loop bomb scenario proposed by Peter et al. (2014). Their physical information derived from the imaging and spectroscopic observations, suggest that they are forming in different plasma environments (heights) in the lower solar atmosphere. 5 IBs are idendified with emission resoponses of O IV, and have been calculated the electron number densities of 5-13 × 10^10 cm^-3, indicating mid chromospheric formation heights. Other IBs without emission response of O IV might form in deep denser plasma. 7 IBs have responses in the intensity images derived from the NUV continuum and the wings of Mg II 2798.8 Å, indicate that the heating heights of these IBs might extend down to lower chromosphere or upper photosphere. We calculate the intensity differences of Si IV 1403 Å to 1394 Å along the wavelength around the position of Ni II 1393.33 Å (the intensity of Si IV 1403 Å has been multiplied by the intensity ratio of Si IV 1394 Å to 1403 Å), and refer to them as the absorption strength of Ni II 1393.33 Å. This method has not been used in the previous studies about IBs. The results suggest that the absorption features of Ni II might be from the cool materials in the chromosphere. In this study, we focus on 12 IBs, which are located in the combined foot-points of multiple transition region loops and might fit with the U-loop bomb scenario. The spectroscopic investigation suggests that they are forming in different plasma environments (heights) in the lower solar atmosphere.