There's already a few threads on the Vicksburg Crater, but in order to make them more accessible I'll post some of the best firsthand accounts of the incident here.
From With Grant at Fort Donelson, Shiloh and Vicksburg by Sgt. Wilbur F. Crummer, 45th Illinois Infantry:
Fort Hill [Third Louisiana Redan] is said to be the key to Vicksburg. We have tried often to turn this key, and have as often failed—in fact, the lock is not an easy one, but we soon shall try the burglar's plan, and with the aid of powder to blow the lock to "smithereens." The sap or trench is run to the fort and the fort is mined, boys digging the dirt and carrying it out in boxes. Great holes are dug underneath the fort, and miners from the Lead Mine, 45th Illinois Regiment, who understand tamping, have charged the 2,200 pounds of powder, and all is ready to light the fuse.
June, the 25th, a heavy artillery fire opened all along the line, and at 2:30 p.m., the explosion takes place. Huge masses of earth were thrown in the air, and the ground was shaken as by an earthquake. As soon as the earth was rent, a bright glare of fire issued from burning powder, but quickly died away, as there was nothing combustible in the fort. A few Confederate soldiers were hurled into the air, one or two of whom came down inside our lines, and some were buried in the fort, as was proven a few years after the war, when the fort was dismantled and turned into a cotton field, a few skeletons were found buried underneath. One negro boy fell among the men of our company. He gathered himself together, and looked around as through he thought the day of judgment had surely come. One of our boys asked him how far up he thought he had gone, and he replied: "Don't know, Massa; 'bout free miles, I guess." He believed it, for I never saw such a frightened look on any one's face, and his eyes stood out and looked unnatural.
When the smoke and dust had cleared away partly, a great saucer-shaped crater was seen, where before was the A-shaped Fort Hill. It was large enough to hold about 60 or 80 men. The 23rd Indiana and the 45th Illinois were in the trenches ready to charge; the command was given before the dust had fully settled; the 23rd Indiana charging to the left of the crater to the top of the works; the 45th Illinois up and into the crater. The enemy had come up behind a big pile of earth thrown out by the explosion, and as we went into the crater, they met us with a terrible volley of musketry, but on the boys went, up and over the embankment with a cheer, the enemy falling back a few paces to an inner or second line of breastworks, where are placed cannon loaded with grape and canister, and these cannon belched forth their death-dealing missiles, in addition to the heavy musketry fire, with such telling effect that may of the brave boys fall to rise no more; the line wavers, staggers, then falls behind into the crater. The enemy charge on us, but we repel them at the west bank of the crater, and a hand-to-hand conflict rages for hours; hand grenades and loaded shells are lighted and thrown over the parapet as you would play ball. These shells and hand grenades carry death, as many as a dozen men being killed and wounded at one explosion. It seems to me, in looking back, a wonder that anyone in that hot place was left to tell the story. I have witnessed our men grab these shells, at the risk of their exploding, and fling them back. Many a brave hero laid down his life in that death hole, or, as we most appropriately called it, "Fort Hell."
The Chicago Tribune had its correspondent in the field and, in the issues of that paper on July 3 and 6, 1863, he speaks of the charge and fighting in the crater, saying : * * * "A wide embrasure in the embankment was made into which the noble Lead Mine Regiment, led by Colonel Maltby, rushed in and at once planted our banner amid a terrific fire from the enemy. The conduct of the 45th Illinois Regiment was grand in the extreme. Universal commendation is bestowed for the gallant manner that regiment performed the duty assigned it, and in no small degree upon the field officers who so nobly inspired the men by taking the advance and marching up to the muzzles of the enemy's guns, so near that for a time it was a hand-to-hand fight. The colors of the regiment planted on the parapet of the fort are literally torn to pieces by the shots of the enemy. Two of the field officers, Lieut. Col. Smith and Major Fisk, are no more. Col. Maltby is still suffering from a severe wound."
We fought at close range with the enemy over that embankment of earth, many of the men receiving bayonet wounds. A cypress log, with port holes cut on the under side, was brought into the crater, and in helping to place it on the parapet, Col. James A. Maltby was severely wounded by splinters from the log. A solid shot from a cannon hit the log, hurling it with terrific force against the Colonel and his small command. Gen. John A. Logan said of Col. Maltby, at the siege of Vicksburg: "He is the bravest man I ever saw on the field of battle." He was in the Mexican War, badly wounded at Chapultepec, then at Fort Donelson in 1862 and then at Vicksburg. He was justly promoted to be a Brigadier General for his bravery.
A detail of about two companies would hold the crater for two hours or more, their rapid firing causing the rifles to become hot and foul, and the men weary and worn out, when two other companies would slip in and take their places. Badeau, in his history of Gen. Grant, says : "Details from Leggett's brigade relieved each other all night long, in their attempt to hold the crater." I want to correct his history and say, as I have a right to say, for I was there and speak from what I know to be the facts, it was no "attempt," it was an accomplished fact that we held it, but to our great loss, until the order was received to give it up.
What a terrible sacrifice it was to hold that little piece of ground. It probably was all right to have made the charge into the crater after the explosion and try to make a breech inside the enemy's lines, but it surely was a serious mistake, either of Gen. Grant or Gen. McPherson, to cause that crater to be held for over 48 hours with the loss of brave men every hour. I remember, upon returning to the trenches, after having been relieved in the crater, of passing Gen. John A. Logan, surrounded by some of his aid-de-camp, and as they bore past him some wounded hero, he broke forth with vehemence, saying: "My God! they are killing my bravest men in that hole." Someone suggested that the place be given up. He said in reply : "I can't; my commanding officer orders me to hold every inch of ground." The crater was at last given up and we resumed the ordinary duties of every-day life in the trenches and in camp.