Sphere

An alien spacecraft is discovered on the floor of the Pacific Ocean, estimated to have been there for 300 years (288). A team of experts, including marine biologist Dr. Beth Halperin, mathematician Dr. Harry Adams, astrophysicist Dr. Ted Fielding, psychologist Dr. Norman Goodman, and U.S. Navy Capt. Harold Barnes, are assembled and taken to the Habitat, a state-of-the-art living environment located near the spacecraft.
On examination of the spacecraft, they determine that it is not alien at all, but rather American in origin. However, its technology far surpasses any in the present day. The ship computer's logs cryptically suggest either a mission originating in the distant past or future, but the team manages to deduce that the long dead crew were tasked with collecting an item of scientific import. Goodman and the others eventually stumble upon a large, perfectly spherical ball of fluid hovering a few feet above the floor in the ship's cargo bay. They cannot find any way to probe the inside of the sphere, and the surface is impenetrable; the crew finds it odd that the surface of the sphere reflects its surroundings except for humans.
They return to the Habitat, and Harry comes to believe that everyone on this team is fated to die. His rationale is that if they survive, their reports will be known by the spacecraft's crew on their future mission, and the crew will be able to foresee and avoid the black hole, thus avoiding the "unknown event," referenced in the logs, and not ending up where Harry's team has found it. During the night, Harry returns to the spacecraft and is able to enter the sphere, then returns to the Habitat. The next day, the crew discovers a series of numeric-encoded messages appearing on the computer screens; the crew is able to decipher them and come to believe they are speaking to "Jerry", an alien intelligence from the sphere. They find Jerry is able to see and hear everything that happens on the Habitat.
A powerful typhoon strikes the surface, and the Habitat crew are forced to stay several more days. During that time, a series of tragedies strike the crew, including attacks from aggressive jellyfish and a giant squid and equipment failures in the base, killing Ted and the team's support staff. The survivors, Beth, Harry, and Norman believe Jerry to be responsible. While waiting for rescue, the three begin to realize that the hazards that the others had befallen were manifestations of their own fears. They all believe that they have entered the sphere, which has given them the ability to make their fears real. Norman discovers that they had misinterpreted the initial messages from Jerry and that the entity speaking to them through the computers is actually Harry himself, transmitted while he is asleep.
Under the stress of the situation, Beth has suicidal thoughts which causes the detonation mechanisms on a store of explosives to engage, threatening to destroy the base and the spacecraft. They race to the Habitat's mini-sub, but their combined fears cause them to appear in the spacecraft. Norman is able to see through the illusion and trigger the mini-sub's undocking process, allowing them to escape the destruction of the Habitat and spacecraft. The sphere is untouched by the explosions.
The mini-sub makes it to the surface as the surface ships return. As Beth, Harry, and Norman begin safe decompression, they realize that they will be debriefed and their newfound powers discovered. They all agree to erase their memories of the event using their powers; this assures that the "unknown event" paradox is resolved. The sphere rises from the ocean and then accelerates off into space.

Question: What does the surface of the sphere not reflect?
Answer: Humans