Yep, after 28+ years another new first. (this was undeveloped land in Central CA near the gold discovery area)
1. Safety, treat every firearm, weapon or bomb as live, loaded and dangerous.
2. Do NOT pick it up.
3. Take photos of High Quality from a safe distance, inform the owners of the next step.
4. Contact the proper authorities, they'll know what to do.
5. Plug your ears, then write a blog about it, oh and respond to all the outlandish scenarios on Facebook as to what caused that loud boom.
Back story, while have land cleared so we could find the well head and proceed with the testing and only remaining contingency prior to closing escrow; the landscaper found a bomb, a very large mortar round 14" x 4". He neglected to mention this to me. Next day while exploring, I come across the round which they had moved to the side of the access road.
I took pictures, surely it was some kind of survey monument, surveyor says no - looks like a bomb. I sent pictures to a firearms specialist, they said military ordinance. So I called the Sheriff and asked if we had a bomb squad (small rural community) We did, but the dispatcher didn't want to forward my call. Said I should take "ammo" to a gun store since they don't deal with it. I insisted that bomb squad call me back. He did 2 hours later, but after I texted the picture, called back in 5 seconds. He sent a deputy to verify then he came up and said it was too big for him to handle and sent pictures to the military "bomb squad". They identified it (a WWI Mortar more than 100 years old) and based on the markings, said back away, it is a live round! 2 Hours later after they determined it was unsafe to move it (remember live round) they asked to detonate it on site. They required us to move away 1/4 of a mile, but when the blast went off, I thought it could have been better to have been farther! It was louder than a sonic boom or any firearm or even tanerite I had every heard and it lit up inquiries on Facebook.
6. Lastly, CA is a full disclosure state, so we had to inform the buyer. Her concerns about how it got there and if it was the only one......I referred her to the local historical society to see if there had ever been military exerises or a firing range.
We are in an interesting business, every day we are presented with new things to learn and new challenges to overcome. Stay safe out there!