Is gravity actually missing or lower in certain spots on Earth? I have heard and read about "missing gravity" in some regions of Earth. What I seemed to gather from this info is that if I go to a certain spot and jump, I can jump higher; or I could maybe lift more weight.
The question is, is this true? Also, can it be explained in great, concrete depth as to why it happens?
I've also heard, "low gravity areas." Are there really certain areas where gravity is weaker to a significant or noticeable measure? Why/how? This one documentary said things could actually be observed of this.
Also, one scientist in the documentary made a claim that gravity in certain spots may get even lower.
 A: First off, a definition of gravitational acceleration from the perspective of a geologist or geophysicist. Imagine a 50 cm tall cylinder with all the air drawn out. A small ball is held in place at the top of the cylinder. The device is firmly fixed to the surface of the Earth and the ball is released. Timing how long it takes for the ball to fall to the bottom of the cylinder yields the local gravitational acceleration: $d=\frac 1 2 g t^2$, or $g = \frac {2d}{t^2}$.
Note that by this definition, acceleration due to gravity includes both gravitational and centrifugal forces. Per this definition, gravitational acceleration at sea level varies from about 9.8337 m/s2 at the north pole to about 9.7803 m/s2 at the equator. Centrifugal force due to the Earth's rotation accounts for about 2/3 of the variation. The other 1/3 is due to the Earth's equatorial bulge.
Another factor that results in variations in gravitational acceleration is altitude. Gravitational acceleration at the top of a mountain should nominally be a bit lower than gravitational acceleration at sea level at the same latitude because the mountaintop is further from the center of the Earth compared to sea level. These altitude variations are much smaller than the ~0.05 m/s2 between the north pole and equator.
Latitude and altitude do not fully explain the variations in gravitational acceleration at the surface of the Earth. What remains is rather small. Using meters per second squared is rather inconvenient to represent these small variations. Geologists and geophysicists instead use the galileo (1 cm/s2). Even this unit is too large to represent the tiny deviations from what one would expect based on latitude and altitude. Geologists and geophysicists use milligals (1/1000 of a galileo) to represent gravity anomalies. The resultant picture:

Source: http://earthobservatory.nasa.gov/Features/GRACE/page3.php
A lot of pop science articles misrepresent the above image. The ±50 milligals variation in the above image is very small compared to the standard gravitational acceleration of 980665 milligals (9.80665 m/s2). It is also very small compared to the 5300 milligals difference between gravitational acceleration at sea level at the north pole versus the equator.
A search engine search for "missing gravity" results in lots of articles about the Hudson Bay area. This is indeed an area where gravitational acceleration is a bit lower than one would expect given the latitude of the area. However, many of those pop sci articles are wrong. Many of those articles claim that gravitational acceleration in the Hudson Bay area is lower than elsewhere on the Earth. This is very wrong. The ~50 milligal negative anomaly in that area is small compared to the much larger positive variation due to the extreme latitude of that area. Gravitation in the reddish region around Indonesia (a positive gravity anomaly) is less than is the gravitation around Hudson Bay.

Finally to answer the question raised in the title of the opening post,

Is gravity actually missing or lower in certain spots on Earth?

Yes, gravity is slightly (very slightly) lower than nominal at certain spots on the surface of the Earth. But no, there are no spots on the surface of the Earth where gravity is "missing".
A: Yep, it is possible to measure this deviation from mean surface gravity from both the earth and space. Take a look at this article on io9:
http://io9.com/new-high-res-maps-of-earth-s-surprisingly-inconsistent-1171851670
There are three main things that cause this.
1) Distance from the centre of the earth. As you get higher, gravitational force becomes measurably less, since gravitational force cuts off as distance squared.
2) Density of the earth under you. Higher density areas of the crust have more mass in a smaller area to attract you.
3) Latitude from the equator. Although this isn't a gravitational effect, the earth's rotation counteracts the force of gravity and this effect is weakest on the polls and strongest at the equator.
All of these effects are very small and would not be noticed by a person on the surface of the earth.
A: Another possible reason for gravitational anomalies.
But, as you ask in your comments: Could this explain why thousands of people disappear each year? No. 
And it would only affect those areas of the world covered by ice to a significant depth in the past. 
Lower than Average Gravity

A new satellite survey may have solved the mystery behind one of the world's strangest weight-loss methods: moving to a large area of northern Canada with unusually low gravity.
  Though the weakness is slight—one part in 25,000, or a tenth of an ounce for a 150-pound (68-kilogram) person—scientists have argued for years about the cause.
One possibility is that underlying mantle rocks are slowly flowing downward.
  It's like being aboard a raft in a rapids, said lead study author Mark Tamisiea, a geophysicist at the Proudman Oceanographic Laboratory in Liverpool, England.
  "If the water was flowing downward in a sinkhole, [the raft] would be pulled down as well," he said.
  But the new study supports an alternate theory: that 20,000 years ago, Ice Age glaciers pressed down on the area's crust like a person sitting on an extremely viscous waterbed.
  The weight of all of that ice forced the mantle rocks to ooze slowly sideways. Then the ice melted—rapidly enough the crust hasn't yet fully bounced back. Tamisiea and colleagues determined that the rebound indeed accounts for about half of the gravity loss. The research appears in today's issue of the journal Science.
The study, conducted at the Harvard-Smithsonian Center for Astrophysics and the University of Toronto, relied on a sensitive pair of NASA satellites named GRACE.
  These satellites follow the same orbit but remain about 130 miles (210 kilometers) apart. Microwaves are used to measure the distance between them with extreme precision. As Earth's gravity varies, so does the distance between the satellites

