Multicenter randomized trial comparing post-procedural pain intensity after Per-Oral endoscopic myotomy (POEM) between two types of standard of care insufflation methods (CO2 vs Underwater). POEM is routinely performed under carbon dioxide insufflation (CO2-POEM) as this gas is more rapidly absorbed than air, which has been shown to reduce gas-related complications. Water immersion for luminal distension of the GI tract as opposed to carbon dioxide insufflation has been shown to be associated with improved patient satisfaction, safety profile and even higher detection of polyps during colonoscopy in randomized trials Aim 1. The primary aim is to compare post-procedural pain following U-POEM vs. CO2-POEM. Aim 2. Compare the proportion of patients that require post-procedural admission for pain management. Aim 3. Compare the need for analgesic medications for pain control in patients undergoing U-POEM vs. CO2-POEM. Aim 4. Compare technical and clinical success between U-POEM vs. CO2-POEM. Technical success is defined as successful completion of the procedure whereas clinical success will be defined as an Eckardt score ≤ 3 at the time of follow-up. Aim 5. Compare and evaluate procedural characteristics between the two groups. 1. Compare procedural time between U-POEM and CO2-POEM. 2. Compare the mean number of coagulations with hemostatic forceps for active intraprocedural bleeding and the mean number of times in which a device besides an electrosurgical knife was required for prophylactic ablation of vessels. 3. Adverse event rate (i.e. bleeding, perforation).