Air carrier flight crew flying B777 aircraft reported a malfunction with the pressurization system. Returned to departure airport and landed uneventfully.

Narrative

While at a cruise altitude of 37;000 ft.; the F/O (First Officer) noticed the appearance of the cabin pressure display. The F/O also noticed that the cabin altitude was 9800 ft. which is not normal. A search for the correct checklist ensued and shortly thereafter the cabin altitude exceeded 10;000 ft. which caused a high cabin altitude warning to annunciate. We donned our 02 masks and established crew communication via the intercom. The cabin altitude uncontrollable checklist was accomplished. I determined that the cabin altitude was uncontrollable based on the observation that both cabin outflow valves were indicating fully closed. I then determined that an expedited descent to 10;000 was required as the next course of action. [Requested priority handling] followed by a request for a descent to 10;000 ft. We were issued a series of step down clearances which resulted in attaining the requested 10;000 in a timely manner. The descent was accomplished using full speed brakes; idle power; and 310 KIAS. I requested holding at the ZZZZZ intersection so as to weigh our options. There was no apparent deficiencies of our aircraft and we had an abundance of fuel. There were no urgencies at this moment. I conferred with Dispatch; Maintenance Control; ATC; and the flight attendants. I made timely PA announcements. It should also be noted that no passenger O2 masks had deployed as we never reached the 14;000 ft. threshold level. Our timely descent precluded that. Prior to reaching 10;000; the IRO was called back from his break and arrived shortly thereafter. He then provided valuable assistance in our planning.While conferring with Maintenance Control via SATCOM; Maintenance Control stated that they desired we would have attempted to manually close the cabin outflow valves. Doing so; would have allowed the valves to close further he stated; possibly allowing the cabin to become controllable again. My own observations indicated the valves to be fully closed. The checklist we used asked a simple question as to whether the cabin was controllable or not.After consultation with Dispatch a decision was made to divert the aircraft back to ZZZ. The trip would be about 2 hours due to the low altitude. We had an abundance of fuel and an aircraft with no apparent defects. The weather at ZZZZ was 1/4 mi. visibility. We would jettison about 34;000 lbs. of fuel in order to be under our maximum landing weight. Exact information regarding the fuel dumping was sent to Dispatch using the text function of ACARS. While returning to ZZZ; I declined most ATC requests for priority handling. I believed that operating in a normal/routine manner was the best course of action. We then began working on low priority issues in regards to passenger comfort and amenities. We landed in ZZZ using autoland with me flying. While deplaning; I spoke with several Maintenance technicians who inspected our aircraft upon arrival. I was shown a picture of one of the external conditioned air inlets. This inlet was stuck in an open position. This would explain a great many things we observed. In my judgment there would be no way way of pressurizing the cabin sufficiently at a typical cruise altitude.I was very pleased with all who assisted us that night.