MINNEAPOLIS -- Last week's intense rainfall in the Twin Cities provided an unexpected bonus to the people working on the playing surface for Target Field.

That rain allowed for the first test of the drainage system that will be in place underneath the grass playing field at the Twins' new ballpark, which is set to open in April of next year.

While the grass turf did not start being installed until Monday, crews had already put in place the complex drainage, heating and irrigation system, as well as the 10-inch top layer of sand and peat, before the very wet weather arrived in Minnesota.

A total of 3 3/4 inches of rain fell within an hour period at the new ballpark in downtown Minneapolis last Wednesday afternoon, creating sheets of water on top of the still-sand field. But within 20 minutes, the entire field was drained of water.

"Even with all that rain, we would have been able to play Wednesday night," said Jerry Bell, president of Twins Sports Inc. "And that's thanks to the entire system working together."

All eyes have been focused on the installation of the grass this week, which is considered a huge milestone for the project. But perhaps it's what is underneath that sod which is the most important aspect of the Twins' new playing field.

Planning for an outdoor field in a place like Minnesota where the weather conditions can be extreme -- from the freezing cold and snow in the winter to the hot, rainy summers -- the Twins made sure to build a field that can withstand both aspects. And so a lot of time and energy was spent in creating a sophisticated system underneath the field to help the grass survive the elements.

To start, here is a breakdown of exactly what is underneath the grass.

Root zone: Just below the grass there is a 10-inch layer called the root zone. It's made up of 93 percent sand and 7 percent peat, providing a solid base for the field.

Heating system: At the bottom of the root zone is the beginnings of a heating system that will help to warm the field in the early spring or October or November if the Twins are still playing at that time. A series of tubes will be filled with water and glycol to make steam to help control the temperature of the ground.

Drainage system: Right under the sand root zone will be a layer of crushed gravel surrounding six-inch drain pipes, which will be placed every 10 feet throughout the playing surface. Trenches have been dug into the existing soil, where each pipe will be placed with all of the gravel around it to drain excess water.

And of course, interspersed in that underlayer throughout the field, is an irrigation system of sprinklers that can pop up to help supply enough water to the grass field and the root zone.

"It makes for a very sophisticated cross-section before you even get to the grass," said Dan Mehls, a Mortenson Construction executive.

All of the fields in modern ballparks have a similar drainage system to the one at Target Field, according to Twins head groundskeeper Larry DiVito. They all allow significant amounts of water to be whisked away from the playing surface.

While the nearly four inches of rain that fell within an hour last week at Target Field is considered a lot for such a small time span, the drainage system will actually be able to allow for as much as 20 inches of water per hour to be cleared from the playing surface in a span of 15-30 minutes.

But perhaps the bigger need in Minnesota is the heating system that will protect the grass over the winter and help to get it ready in time for Opening Day.

DiVito said that while heating tubes run underneath the entire playing field, the system is divided into five "heat zones." It allows DiVito to control the temperature of just one section or the entire field if he'd like.

The heating system will be used over this winter to keep the ground from completely freezing under the grass, as DiVito and his team will monitor the temperature through how much steam is run through the tubing.

"We can allow frost to just go down to a certain depth or not go down at all," said Mehls. "And we can slowly turn [the heat] up higher in the spring to trick the grass into coming out of hibernation earlier than it normally would."

For DiVito, it will be a little trial and error to see how best to use the heating system and blankets covering the grass to get it in the best condition. DiVito certainly has a lot of experience, having spent three years in Washington D.C. as the Nationals' head groundskeeper and four years as the No. 2 groundskeeper for the Dodgers.

This will be a new climate for him to work with in Minnesota, but it's not completely new as he was in charge of the field for the Triple-A Pawtucket Red Sox for seven years.

"Working in New England with no heat was a challenge," DiVito said. "The way the sun angled in late March, there was no sun in the batters' box while we were trying to thaw the clay. So I've been down in the trenches before."

But this time, DiVito will have just a few more tools at his advantage.