Inform the police. They will bring bomb experts to defuse it.

Inform the police or local authorities and let them deal with it. That size of weapon is enough to severely damage the area around it so you don't want to go near it. It was also customary in the second world war to fit booby traps to such bombs incase the main detonator didn't go off (sometimes one was never installed intentionally), so don't go playing with it or touching it as you could make it go off. I would seriously advise not trying to open/move/diffuse it.

"At 20.52 on this evening a stick of bombs fell on North Shields, two major fires being started. Seventeen minutes later the attack was carried to South Shields when a 1,000kg bomb fell in the garden behind the Simonside Arms on the Newcastle Road. A crater 50' by 15' was made and 90' of a wall was demolished, but the explosion only caused superficial damage to the hotel and some breakage of windows nearby. There were no casualties. At the same time two bombs fell in Commercial Road; one struck the foundations of an old house on the bankside below Smith's Foundry, ricocheted, passed through the exterior wall of a house on the opposite side of the road and came to rest on the floor of an upstairs room between a partition wall and a bed, failing to explode. In the course of its flight the base plate and fins of the bomb were broken off but it weighed 500 lbs. without them. It was removed by the Bomb Disposal Squad the next day. The other bomb hit Holy Trinity School, completely wrecking the buildings and blocking the road with debris. A gas main was burst, the windows in the Holy Trinity Church were shattered and structural damage was done to other buildings in the vicinity. A few minutes later, a 1,000kg bomb fell immediately behind 4 houses in Harton Lane. These houses were reduced to rubble, and covered a frontage of 86'. There was 1 fatal casualty, a woman who had remained in her house during the raid, the other occupants of the houses, who had gone to their Anderson Shelters, were unharmed. A gas main and a water pipe were fractured. Other houses in the neighbourhood were damaged. At Harton Dye Works the main buildings were completely wrecked by another 1,000kg bomb which fell in the centre of the works; all the heavy machinery in the laundry was destroyed and much harm done to surrounding departments, necessitating further demolition. There were no casualties. 3 more bombs fell on the fields near Prince Edward Road, 1 between the Royal Engineers hutments and the searchlight battery on Brinkburn Farm field. There was no damage to the huts or the searchlights but a soldier on sentry duty was injured. The other 2 bombs fell harmlessly in fields." Source and further information: http://www.bpears.org.uk/NE-Diary/Inc/ISeq_23.html I assume you have already told the authorities about it...

you will have the great start of a hole for a swimming pool. It will probably wake the neighbors for several miles around, so don't hit it with a hammer until after 10AM. Call the UXB. they will handle it or give you coffee and doughnuts and tell you, you are safe.

The first answer covers the damage potential. A 2k fuzed bomb can do major damage. Call the authorities immediately and make a hotel reservation. They're used to these situations. Isn't war lovely? At least it wasn't a mine or something.

Fun, I was on duty in Germany when we found one at a US Military Base during road construction. We had a day of angry commuters and lots of lookie-loos trying to get a peak. Give us an update as I assume you have had it removed by now. I am confident it was a memorable show for your whole neighborhood. How long were you out of your house before it was removed... ... that is unless you discovered your septic tank by mistake.